U.S. patent application number 10/146006 was filed with the patent office on 2003-11-20 for process and device for controlling the thermal and electrical output of integrated micro combined heat and power generation systems.
Invention is credited to Anson, Donald, Coll, John Gordon, Hanna, William Thompson, Lambert, John Edward, Stickford, George Henry, Yates, Jan Beryl, Zaborski, Raymond.
Application Number | 20030213246 10/146006 |
Document ID | / |
Family ID | 29418720 |
Filed Date | 2003-11-20 |
United States Patent
Application |
20030213246 |
Kind Code |
A1 |
Coll, John Gordon ; et
al. |
November 20, 2003 |
Process and device for controlling the thermal and electrical
output of integrated micro combined heat and power generation
systems
Abstract
Microprocessor-based control sub-systems which control the
thermal and electrical output of integrated micro-combined heat and
power generation (M-CHP) systems used to supply domestic electrical
power, domestic space heating (SH) water, and domestic hot water
(DHW). The M-CHP system uses a microprocessor controller to control
the internal operating conditions, such as pump speeds, gas flow
rate, and evaporator outlet temperature. Controlling these
parameters enables setting the capacity of the system at any
instant in time, thereby permitting load following, using a
variable capacity operation. The controller also monitors through
sensors a number of additional safety controls and system
protection devices, such as relays/contactor of the alternator to
grid, and electrical trips to the feed pump, the oil pump, the
hydronic pump, the blower, the gas valve, the expander bypass
valves, and other electrically powered devices in the system.
Inventors: |
Coll, John Gordon;
(Somerset, OH) ; Hanna, William Thompson;
(Gahanna, OH) ; Anson, Donald; (Worthington,
OH) ; Stickford, George Henry; (Dublin, OH) ;
Lambert, John Edward; (Dublin, OH) ; Yates, Jan
Beryl; (Reynoldsburg, OH) ; Zaborski, Raymond;
(Westerville, OH) |
Correspondence
Address: |
Killworth, Gottman, Hagan & Schaeff, L.L.P.
Suite 500
One Dayton Centre
Dayton
OH
45402-2023
US
|
Family ID: |
29418720 |
Appl. No.: |
10/146006 |
Filed: |
May 15, 2002 |
Current U.S.
Class: |
60/653 ;
60/670 |
Current CPC
Class: |
F01K 17/02 20130101;
F01K 25/08 20130101; F24H 2240/00 20130101; Y02E 20/14
20130101 |
Class at
Publication: |
60/653 ;
60/670 |
International
Class: |
F01K 007/34; F01K
001/00 |
Claims
We claim:
1. A control subsystem for governing the operation of a
cogeneration system configured to operate with an organic working
fluid, said system having a plurality of functional devices
including at least a heat source, an expander having operatively
coupled to a generator to produce electricity, a condenser in fluid
communication with said expander and adapted to heat a hydronic
heating system, and a pump configured to circulate said organic
working fluid from said condenser through piping in thermal
communication with said heat source such that heat transferred
therefrom superheats said organic working fluid to provide
superheated organic working fluid vapors to said expander, said
control subsystem comprising a programmed processor operatively
coupled at least to said pump and said heat source, and adapted to
operate said pump and said heat source in response to a call for
heat, causing said organic working fluid in said piping in thermal
communication with said heat source to be superheated and provided
to said expander.
2. The control sub-system according to claim 1, wherein said
functional devices of the cogeneration system further include a
power grid contactor adapted to connect the electrical power output
of said generator with a power grid and an electrical load, wherein
said controller is further operatively coupled to said generator
and said contactor for receiving and monitoring electrical power
output from said generator, and connecting/disconnecting said
electrical power output with said electrical load.
3. The control subsystem according to claim 1, further comprising a
plurality of sensors each providing a sensor signal indicative of a
parameter of the cogeneration system, said controller coupled to
said plurality of sensors and adapted to at least monitor the
operation of said functional devices of the cogeneration
system.
4. The control subsystem according to claim 3, further comprising
an operator interface coupled to said controller enabling
modification of the operation of said controller in monitoring said
functional devices and operating characteristics of the
cogeneration system through entry of information via said operator
interface.
5. The control subsystem according to claim 4, wherein said
operator interface includes a data acquisition and communications
subsystem enabling data logging and reporting of said operating
characteristics of said functional devices of the cogeneration
system.
6. The control subsystem according to claim 5, wherein said a data
acquisition and communications subsystem includes a modem for
reporting at least alarm conditions of said cogeneration
system.
7. The control subsystem according to claim 3, wherein said
parameters include at least three of condenser inlet temperature,
condenser outlet temperature, hydronic fluid flow, hydronic supply
temperature, expander inlet temperature, expander inlet pressure,
feed pump inlet temperature, feed pump inlet pressure, power module
power output, evaporator inlet temperature, gas flow, expander
outlet temperature, outdoor ambient temperature, feed pump speed
(drive frequency), protection relay trip, level 1 trip failure, and
level 2 trip failure (failed to re-start).
8. The control sub-system according to claim 3, further comprising
a plurality of control points, said controller being operatively
coupled to said control points of said cogeneration system
associated with functional devices thereof, said controller
responding to said sensor signals to control said functional
devices to thereby vary operating conditions of the cogeneration
system.
9. The control sub-system according to claim 3, wherein said
controller operates under program control for acquiring said sensor
signals and generating control signals for application to said
functional devices of said cogeneration system.
10. The control sub-system according to claim 9, wherein said
controller operates according to said program control and at least
acquires and takes into account an outdoor ambient temperature
reading from one of said sensors before generating said control
signals.
11. The control sub-system according to claim 10, wherein said
controller determines a set-point for a hydronic supply temperature
in the hydronic heating system from said outdoor ambient
temperature reading.
12. The control sub-system according to claim 11, wherein said
controller establishes said set-point according to a linear scale
from 25.degree. C. at an outdoor temperature of 20.degree. C. to
75.degree. C. at an outdoor temperature of -20.degree. C.
13. The control sub-system according to claim 11, wherein the heat
source comprises a gas valve and a burner, and wherein said
controller uses said set-point to operate the heat source in a
variable capacity mode by modulating the gas valve on the burner to
maintain actual hydronic supply temperature as sensed by one of
said sensors at said set-point.
14. The control sub-system according to claim 13, wherein said
controller further prevents working fluid in the liquid state from
entering the expander by coordinating a fuel flow rate (heat input
rate) to the burner with a feed flow rate of the working fluid
exiting the feed pump.
15. The control sub-system according to claim 14, wherein the
functional devices further include an evaporator fluidly connected
to said expander by said piping and heated by said heat source, and
said controller controls said fuel flow rate and said feed flow
rate to maintain an exit temperature of the evaporator at
310.degree. F. (154.4.degree. C.).
16. The control sub-system according to claim 1, wherein the
functional devices further include a hydronic pump circulating
fluid in the hydronic heating system to and from heat exchange
piping of the condenser, and wherein said controller controls the
hydronic pump at a speed that maintains a pressure difference
between the supply and return of the fluid to the heat exchanger
piping of the condenser at a preselected value for optimal
thermodynamic performance of the hydronic heating system.
17. The control sub-system according to claim 1, wherein the
functional devices further include an evaporator fluidly connected
between the feed pump and expander, and a desuperheater fluidly
connected between the expander and condenser, said desuperheater
having a return to the evaporator, and a switching valve provided
in the piping for directing the organic working fluid either to the
desuperheater or the evaporator directly, and wherein the
controller controls the switching of the switching valve.
18. The control sub-system according to claim 1, wherein the
expander is selected from a positive displacement expander and a
scroll expander.
19. The control sub-system according to claim 1, wherein the
functional devices further include a bypass valve and a shutoff
valve in the piping to bypass and shutoff said superheat working
fluid vapors from entering into the expander, the piping including
a bypass loop connected between the bypass valve and condenser, and
wherein said controller controls the opening/closing of the bypass
valve and shutoff valve at least in response to startup and
shutdown conditions of the cogeneration system.
20. The control sub-system according to claim 19, wherein said
shutdown conditions include a heat call satisfied signal, a startup
sequence failure, or exceeding a related preset value for an
expander inlet temperature, an expander inlet pressure, a feed pump
inlet temperature, a feed pump inlet pressure, a protection relay
trip, or a power module temperature.
21. A control subsystem for governing the operation of a
cogeneration system configured to operate with an organic working
fluid, said system having a plurality of functional devices at
least including a heat source, an expander having operatively
coupled a generator to produce electricity, a condenser in fluid
communication with said expander, and a pump configured to
circulate said organic working fluid from said condenser through
piping in thermal communication with said heat source such that
heat transferred therefrom superheats said organic working fluid to
provide superheated organic working fluid vapors to said expander,
said control subsystem comprising: a plurality of sensors each
providing a sensor signal indicative of a parameter of the
cogeneration system; a plurality of control points; and a
programmable controller coupled to said plurality of sensors and to
said control points of said cogeneration system associated with
functional devices thereof, said controller responding to said
sensor signals to control said functional devices of the
cogeneration system to thereby vary the operating characteristics
of the cogeneration system.
22. A control system in combination with a cogeneration system
having a plurality of functional devices and using an organic
working fluid to heat a hydronic heating system and produce
electrical power, the combination comprising: a plurality of
sensors for providing electrical sensor signals indicative of
operating parameters of the cogeneration system; a plurality of
control points associated with said functional devices to change
the operating parameters of the cogeneration system; and a
programmable controller coupled to the electrical sensors and to
said control points of the system associated with said functional
devices, said controller responding to the sensor signals and
generating a plurality of control signals to control the operation
of the functional devices; said controller defining a plurality of
interactive control loops each generating a control signal for
controlling a different one of said functional devices as a
function of variation in a sensor signal supplied to the controller
relative to at least a set-point value for a control loop for the
hydronic heating system, wherein the set-point is determined by
said controller from receiving an outdoor ambient temperature
sensor signal from one of said sensors.
23. A method of controlling the thermal and electrical output of
integrated micro-combined heat and power generation systems used to
supply domestic electrical power, domestic space heating (SH)
water, and/or domestic hot water (DHW), and which converts heat
energy contained in superheated vapors of an organic working fluid
to mechanical energy, and distributes the superheated vapors under
pressure to at least one functional device having a heating need
which varies over time, comprising: monitoring over a period of
time an ambient outdoor temperature to determine said heating need
by said at least one functional device; and changing in response to
said ambient outdoor temperature indicating at any given time that
a different amount of said superheated vapors than that being
delivered to said at least one functional device is so needed to
satisfy said heating need.
24. The method according to claim 23, wherein there are a plurality
of process control sensors communicating with a controller
programmed to change operating parameters of said system in
response to said ambient outdoor temperature in order to furnish
said at least one functional device a supply of said superheated
vapors.
25. The method according to claim 24, wherein said ambient outdoor
temperature is used to determine a desired hydronic supply
temperature for a hydronic fluid and said amount of superheat
vapors is varied to heat said hydronic fluid to said desired
hydronic supply temperature.
26. The method according to claim 25, further comprising sensing an
actual hydronic supply temperature via at least one of said process
control sensors, and adjusting a firing rate of a burner used to
heat said organic working fluid in order to bring said actual
hydronic supply temperature into line with said desired hydronic
supply temperature.
27. The method according to claim 26, further comprising increasing
said firing rate if said actual hydronic supply temperature is too
low, and decreasing said firing rate if said actual hydronic supply
temperature is too high.
28. The method according to claim 26, further comprising sensing
via at least one of said process control sensors temperature of
said superheated vapors and adjusting flow rate of said working
fluid past said burner to maintain said temperature of said
superheated vapors within a desired operating parameter.
29. The method according to claim 28, wherein said desired
operating parameter for said temperature of said superheated vapors
is 310.degree. F. (154.4.degree. C.).
30. The method according to claim 28, wherein said flow rate is
increased if said temperature of said superheated vapors is above
said desire operating parameter, and decreased if said temperature
of said superheated vapors is below said desired operating
parameter.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to cogeneration systems and,
more particularly, to microprocessor-based control sub-systems for
controlling the thermal and electrical output of integrated
micro-combined heat and power generation systems used to supply
domestic electrical power, domestic space heating (SH) water, and
domestic hot water (DHW).
[0002] The concept of cogeneration, or combined heat and power
(CHP), has been known for some time as a way to improve overall
efficiency in energy production systems. With a typical CHP system,
heat (usually in the form of hot air or water) and electricity are
the two forms of energy that are generated. In such a system, the
heat produced from a combustion process can drive an electric
generator, as well as heat up water, often turning it into steam
for dwelling or process heat. Most present-day CHP systems tend to
be rather large, producing heat and power for either a vast number
of consumers or large industrial concerns. Traditionally, the
economies of scale have prevented such an approach from being
extrapolated down to a single or discreet number of users. However,
increases in fuel costs have diminished the benefits of
centrally-generated power. Accordingly, there is a potentially
great market where large numbers of relatively autonomous,
distributed producers of heat and electricity can be utilized. For
example, in older, existing heat transport infrastructure, where
the presence of fluid-carrying pipes is pervasive, the inclusion of
a system that can provide CHP would be especially promising, as no
disturbance of the adjacent building structure to insert new piping
is required. Similarly, a CHP system's inherent multifunction
capability can reduce structural redundancy.
[0003] The market for localized heat generation capability in
Europe and the United Kingdom (UK), as well as certain parts of the
United States, dictates that a single unit for single-family
residential and small commercial sites provide heat for both SH
(such as a hydronic system with radiator), and DHW (such as a
shower head or faucet in a sink or bathtub), via demand or
instantaneous system. Existing combination units are sometimes
used, where heat for DHW is accumulated in a combination storage
tank and boiler coil. In one configuration, SH water circulates
through the boiler coil, which acts as the heating element for the
water in the storage tank. By way of example, since the storage
capacity required for instantaneous DHW supplying one to two
showers in a single family residence (such as a detached house or a
large apartment) is approximately 120 to 180 liters (roughly 30 to
50 gallons), it follows that the size of the storage tank needs to
be fairly large, sometimes prohibitively so to satisfy thermal
requirements of up to 25 kilowatts thermal (kWt) for stored hot
water to meet such a peak shower demand. However, in newer and
smaller homes there is often inadequate room to accommodate such
storage tank volume. In addition to the need for instantaneous DHW
capacity of up to 25 kWt, up to 10 kWt for SH is seasonally needed
to heat an average-sized dwelling.
[0004] Furthermore, even in systems that employ SH and DHW in a
single heating system to consolidate spacing, no provision for CHP
is included. In the example given above, it is likely that the
electrical requirements concomitant with the use of 35 kWt will be
between 3 and 5 kilowatts electric (kWe). The traditional approach
to providing both forms of power, as previously discussed, was to
have a large central electricity generating station provide
electricity on a common grid to thousands or even millions of
users, with heat and hot water production capacity provided at or
near the end-user on an individual or small group basis. Thus, with
the traditional approach, the consumer has not only little control
over the cost of power generation, as such cost is subject to
prevailing rates and demand from other consumers, but also pays
more due to the inherent inefficiency of a system that does not
exploit the synergism of using otherwise waste heat to provide
either additional electric generation or heating capacity.
[0005] Large-scale (in the megawatt (MW) range and up) cogeneration
systems, while helpful in reducing the aforementioned
inefficiencies of centrally-based power generation facilities, are
not well-suited to providing small-scale (below a few hundred kW)
heat and power, especially in the small-scale range of a few kWe
and below (micro-based systems) to a few dozen kWe (mini-based
systems). Much of this is due to the inability of the large prime
mover systems to scale down, as reasonable electrical efficiency is
often only achieved with varying load-responsive systems, tighter
dimensional tolerances of key components, and attendant high
capital cost. Representative of this class are gas turbines, which
are expensive to build for small-scale applications and which
sacrifice efficiency when operating over varying electrical load
requirements. Efficiency-enhancing devices, such as recuperators,
tend to reduce heat available to the DHW or SH loops, thus limiting
their use in high heat-to-power ratio (hereinafter Q/P)
applications.
[0006] A subclass of the gas turbine-based prime mover is the
microturbine, which includes a high-speed generator coupled to
power electronics. This microturbine could be a feasible approach
to small-scale cogeneration systems. Other shortcomings associated
with large-scale CHP systems stem from life-limited configurations
that have high maintenance costs. This class includes prime movers
incorporating conventional internal combustion engines, where
noise, exhaust emissions, lubricating oil and spark plug changes
and related maintenance and packaging requirements render use
within a residential or light commercial dwelling objectionable.
This class of prime mover also does not reject a sufficient amount
of heat for situations requiring a high Q/P, such as may be
expected to be encountered in a single family dwelling. Other prime
mover configurations, such as steam turbines, while generally
conducive to high Q/P, are even less adapted to fluctuating
electrical requirements than gas turbines. In addition, the
steam-based approach typically involves slow system start-up, and
high initial system cost, both militating against small-scale
applications.
[0007] In view of the limitations of the existing art, the
inventors of the present invention have discovered that what is
needed is an autonomous system that integrates electric and heat
production into an affordable, compact, efficient and distributed
power generator, and the ability to control the thermal and
electrical output of such a system.
SUMMARY OF THE INVENTION
[0008] The above mentioned needs are meet by providing
microprocessor-based control sub-systems which control the thermal
and electrical output of integrated micro-combined heat and power
generation (M-CHP) systems used to supply domestic electrical
power, domestic space heating (SH) water, and domestic hot water
(DHW). The M-CHP system uses a microprocessor controller to control
the internal operating conditions, such as pump speeds, gas flow
rate, and evaporator outlet temperature. Controlling these
parameters enables setting the capacity of the system at any
instant in time, thereby permitting load following, using a
variable capacity operation. The controller also monitors a number
of additional safety controls and system protection devices, such
as relays/contactor of the alternator to grid, and electrical trips
to the feed pump, the oil pump, the hydronic pump, the blower, the
gas valve, the expander bypass valves, and other electrically
powered devices in the system.
[0009] Overall, system capacity control emphasizes simplicity,
reliability, and low cost. For the most part, the most basic
control system is a single flow rate system with on/off control
from a space thermostat, wherein the instantaneous capacity of a
hydronic heating system of M-CHP system is fixed. In a more
preferred system, working fluid flow rate and system thermal
capacity are ultimately determined by the processor reading the
outdoor ambient temperature. The outdoor ambient temperature is
used to determine a set-point for the supply (hot-water)
temperature in the hydronic heating system. The controller uses a
look-up table or algorithm to establish the set-point according to
a linear scale, such as varying the set-point linearly from
25.degree. C. at an outdoor temperature of 20.degree. C. to
75.degree. C. at an outdoor temperature of -20.degree. C. The
controller uses this set-point to operate the M-CHP in a variable
capacity mode by modulating the pump flow rate to maintain the
actual hydronic supply temperature at the desired set-point.
[0010] Additionally, the controller coordinates the burner fuel
flow rate (heat input rate) with the feed pump flow rate to provide
optimal thermodynamic performance by preventing liquid from
entering the expander. This is accomplished by maintaining a fixed
evaporator exit temperature, such as, for example, 310.degree. F.
Furthermore, the controller controls the hydronic pump of the
heating system at a speed that keeps the pressure difference
between the supply and return headers of the condenser at a
preselected value for optimal thermodynamic performance of the
hydronic heating system.
[0011] In normal operation the M-CHP system capacity will vary as
needed to maintain the supply temperature at its set-point, wherein
the burner operates to provide the needed heat input. System mass
flow varies essentially with pump speed, but since the expander
operates at nearly a fixed speed, the volume flow of high-pressure
vapor is essentially constant. Thus, the evaporator pressure varies
with load, allowing the vapor density at the evaporator exit to
vary to maintain a constant volume flow at each mass flow setting.
Preferably, a variable frequency drive is used for the feed pump
motor. However, a motor with discrete speed steps may also be used,
as well as any other variable speed or flow approach. In normal
operation the system operates at higher capacity to attain the
higher hydronic supply temperatures required by colder weather.
With the higher hydronic supply temperature, the capacity of the
indoor radiators/convectors increases to handle the added heat
flow. After M-CHP shut down, the hydronic pump may run on to
deliver residual heat in the hydronic system to the radiators and
maintain the flow of valid control data to the controller from the
hydronic system.
[0012] In one embodiment, coordinating the flow rates consists of
open loop speed control of the feed pump with an induction motor.
Proportional integral differential (PID) control of the boiler gas
flow to maintain an outlet superheat between about 20.degree. F.
and about 30.degree. F. using a closed loop control system may also
be provided. A liquid condensate reservoir ahead of the feed pump
and the liquid level is maintained above a predetermined level to
ensure adequate net positive suction head (NPSH) to the pump.
Preferably, no active control is necessary as a sufficient minimum
level can be maintained under all operating conditions by filling
the loop with a minimum refrigerant quantity for most operating
conditions. However, if for certain operating conditions, the
liquid level at the pump inlet cannot be maintained within a
desired range, then an active means of maintaining liquid level
control may be used. For example, level sensing at the liquid
reservoir along with some means of speed control of the feed pump
motor may be used.
[0013] High and low side pressures float depending mainly on boiler
heat input and hydronic loop return temperature to the condenser.
Additionally, no active control of the power module speed is
necessary. Connection of the generator to the grid loads the
alternator sufficiently to limit the speed to about 3150 RPM (50 hz
application). However, should the controller sense a power module
overspeed (e.g., loss of grid connection, by sensing, for example,
the rate of change of frequency), it will quickly command the
opening of a bypass loop around the expander. Preferably, the
sensor to detect expander overspeed is built into the power
module.
[0014] Typically, it is unnecessary for the controller to control
the generator output as long as the generator output is less than
the dwelling load. However, the controller can be programmed to
monitor and deal with the situation when that generator output does
exceeds dwelling load. In particular, the controller can be
programmed to switch the extra power on the public grid or use the
extra power for a resistance heater to preheat boiler feed.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0015] The following detailed description of the preferred
embodiments of the present invention can be best understood when
read in conjunction with the following drawings, where like
structure is indicated with like reference numerals and in
which:
[0016] FIG. 1 shows a schematic diagram of an integrated micro-CHP
system including a control subsystem according to the present
invention;
[0017] FIG. 2 shows a schematic diagram of a PLC control subsystem
according to one aspect of the present invention;
[0018] FIG. 3 is a start-up/operating flow chart of the operating
logic of one embodiment of the integrated micro-CHP system
according to the present invention;
[0019] FIG. 4 is a shutdown flow chart of the operating logic of
the integrated micro-CHP system according to one embodiment of the
present invention;
[0020] FIG. 5 shows a schematic diagram of a sensor that may be
used by a control subsystem of one embodiment of the present
invention to sense the saturation pressure of an organic working
fluid of an integrated micro-CHP system;
[0021] FIG. 6 is a chart showing measured outside ambient
temperature versus desired hydronic set point; and
[0022] FIG. 7 is a chart showing net power output at various
hydronic supply temperatures, which varies approximately linearly
with mass flow.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0023] With reference to the figures, an embodiment of the
invention is illustrated incorporated into a number of the major
components of an illustrated organic rankine cycle (ORC)
micro-cogeneration system used for the supply of domestic
electrical power, domestic space heating (SH) water, and domestic
hot water (DHW). Fluid supply lines connecting these components
together are not drawn for ease of illustration. However, it is to
be understood that the present control subsystems and control
method of the present invention may advantageously be used with a
number of micro-cogeneration systems. Exemplary micro-cogeneration
systems are disclosed by co-owned U.S. patent application Ser. No.
09/998,705 filed Nov. 30, 2001, the entire disclosure of which is
herein incorporated fully by reference.
[0024] Referring initially to FIG. 1, a number of system components
of an exemplary micro-CHP system 100 is schematically shown, such
as an expander 101, a condenser (heat exchanger) 102, feed pump
103, heat source (boiler) 104, and evaporator 105. The expander 101
may be of any design, such as for example, a positive displacement
expander and a scroll expander. In a direct fired system, the feed
pump 103 circulates an organic working fluid (such as
naturally-occurring hydrocarbons or halocarbon refrigerants, not
shown) through a loop at least defined by the fluidly-connected
expander 101, condenser 102, feed pump 103 and evaporator 105. In
an indirectly fired system, the feed pump 103 circulates the
working fluid through a first or inner loop, and at least defined
by fluidly-connected expander 101, condenser 102, feed pump 103,
and a first loop portion of evaporator (heat exchanger) 105, which
is illustrated by dashed-line 105a. In such an arrangement, the
first loop portion or interloop portion 105a receives heat from a
second loop portion of the evaporator or heat pipe which is
directly heated by heat source 104. More detailed information is
provided in co-pending U.S. patent application Ser. No. 09/998,705
regarding directly and indirectly micro-CHP system arrangements, to
which reference is made.
[0025] Optionally, a desuperheater 106 may be included between the
outlet of the expander 101 and the inlet of condenser/heat
exchanger 102. In this alternative embodiment, a regulated by-pass
valve 107 is used to circulate the working fluid exiting feed pump
103 through the coil loops (not shown) of the desuperheater 106
extracting heat energy from the vaporized working fluid exiting
expander 101 before being circulating into evaporator 105 or
interloop portion 105a, if so configured. Since the size of the
desuperheater is fixed, the amount of heat removed from the working
fluid vapor exiting expander 101 will vary with working fluid flow
rate and temperature as the system modulates. However, it is to be
appreciated that controllable by-pass valve 107 permits the ability
to control the flow of liquid working fluid through the
desuperheater. By controlling the liquid working fluid flow, the
amount of de-superheating is controllable, and therefore the
heat-to-power ratio of the micro-CHP is controllable.
[0026] When maximum thermal output is required, by-passing the
desuperheater 106 will divert all the thermal energy to the
hydronic fluid in condenser 102. When maximum electrical power
output is desired, directing all the liquid refrigerant coolant
first to the desuperheater 106 will remove the maximum amount of
energy from the working fluid vapor in the desuperheater prior to
it entering the condenser 102. By controlling the liquid working
fluid flow rate between zero and 100 percent mass flow to the
desuperheater 106, the thermal and electrical outputs can be
tailored to better match the space heating load and the electrical
power load of the house or building.
[0027] A generator 108 (preferably induction type) is coupled to
expander 101 such that motion imparted to it by expander 101
generates electricity. While the expander 101 can be any type, it
is preferable that it be a scroll device. The scroll expander can
be a conventional single scroll device, as is known in the art. The
generator 108 is preferably an asynchronous device, thereby
promoting simple, low-cost operation of the system 100, as complex
generator speed controls and related grid interconnections are not
required. An asynchronous generator always supplies maximum
possible power without controls, as its torque requirement
increases rapidly when generator 108 exceeds system frequency. The
generator 108 can be designed to provide commercial frequency
power, 50 or 60 Hz, while staying within close approximation (often
150 or fewer revolutions per minute (rpm)) of synchronous speed
(3000 or 3600 rpm). The load on the expander 101 imposed by the
grid ensures that mechanical speeds in the expander 101 are kept
within its structural limits.
[0028] Inherent in a micro-CHP (cogeneration) system is the ability
to provide heat in addition to electricity. Excess heat, from both
the heat source 104 and the expanded working fluid, can be
transferred to external DHW and SH loops. The nature of the heat
exchange process is preferably through either counterflow heat
exchangers (e.g., for either or both the DHW and SH loops), or
through a conventional hot water storage tank (e.g., for a DHW
loop). In one embodiment, for example, a simple SH loop 109 may
include fan 110 which provides for the drawing and blowing of air
heated by heat source 104 to a space 111. For DHW services, an
external heating loop 112 (shown partially) may be coupled to
condenser 102. As an option, a preheat coil (not shown) can be
inserted into the external heating loop 112 such that the hydronic
fluid (typically water) flowing therethrough can receive an
additional temperature increase by virtue of its heat exchange
relationship with the heat exchange fluid flowing through a second
circuit (not shown).
[0029] The hydronic fluid flowing through external heating loop or
DHW system 112 is circulated with a conventional hot water pump
113, and is supplied as space heat via a radiator or related device
(not shown) to, for example, space 111. As an example, hydronic
fluid could exit the condenser 102 at about 50.degree. Celsius and
return to it as low as 30.degree. Celsius. It will be appreciated
by those of ordinary skill in the art that while the embodiments
depicted in the figures show separate DHW and SH heat exchangers,
it is within the spirit of the present disclosure that series,
parallel, sequential and/or the same heat exchange configurations
could be used. Additionally, although the capacity of the system
100 is up to 60 kW.sub.t if desired, larger or smaller capacity
units could be controlled by the herein disclosed control subsystem
and method of the present invention.
[0030] Combustion chamber 114, enclosing heat source 104, includes
at least a flumed exhaust duct 115 to the outside of the building,
and an exhaust gas fan 116. Heat at least to the evaporator 105 is
provided by heat source or burner 104, which is supplied with fuel
by a gas train 117 having a shut off valve 118 and variable flow
gas valve 119.
[0031] Other devices may be used with the exhaust duct 115, such as
an exhaust gas recirculation device with an exhaust duct heat
exchanger (not shown) which can be used to improve the thermal
efficiency of the system 100 by lowering the temperature of the
exhaust gas that is pulled away and vented to the atmosphere by fan
116. The heat given up by the exhaust gas in an exhaust gas heat
exchanger may be used to provide additional heat to other parts of
the system 100. More detailed information is provided by co-pending
U.S. patent application Ser. No. 09/998,705 regarding exhaust gas
recirculation devices with or without an exhaust duct heat
exchangers, to which reference is made.
[0032] Block valve 120 and bypass valve 121 are situated in the
organic working fluid flow path defined by piping 122 (of which
bypass conduit 123 is part). In response to a no load condition
(such as a system start up/shut down or grid outage) on the system,
the superheated vapor in piping 122 is permitted to bypass around
expander 101 through bypass conduit 123, thereby avoiding overspeed
of expander 101. In this condition, the rerouted superheated vapor
is fed into the inlet of condenser 102 or if so configured,
desuperheater 106. Under normal operating conditions, where there
is a load on the system, the superheated vapor enters the expander
101, causing the orbiting involute spiral wrap to move relative to
the intermeshed fixed involute spiral wrap. As the superheated
vapor expands through the increasing volume crescent-shaped
chambers of the expander 101, the motion it induces in the orbiting
wrap is transferred to the generator 108, via a coupled shaft or an
integral rotor/stator combination on the expander 101.
[0033] Controller Sub-Systems
[0034] As illustrated by FIG. 2, provided are controller
sub-systems, generally indicated by 124, to control the thermal and
electrical output of integrated micro-combined heat and power
generation (M-CHP) systems, such as illustrated by FIG. 1. The
controller sub-system 124 are microprocessor-based, wherein the
M-CHP system 100 uses a microprocessor controller 125, which is
preferably a programmable logic controller (PLC) or alternatively,
any other conventional microcomputer, to monitor and control the
operating conditions of system 100. It is to be appreciated that a
PLC based subsystem simplifies setup and troubleshooting and may
accommodate programming modifications from future developments and
refinements to such micro-CHP systems. In a preferred embodiment,
the controller is an Allen Bradley MicroLogix 1500 PLC with a
1764-LRP Processor and includes other components, such as for
example, shown below in Table 1 for it operation, wherein the
controller 125 is programmed using conventional programming
software and techniques. In a preferred embodiment, the controller
125 ladder logic may be programmed with Allen Bradley RS Logix 500
software, wherein a graphical maintenance program running Allen
Bradley RS View on a laptop PC can be used for troubleshooting and
setup in the field.
[0035] The controller subsystems 124 further include a plurality of
process control sensors 126, a plurality of control outputs 127, a
protection relay subsystem 128, a system trip reset 129, and the
necessary user and service personnel interfaces, such a maintenance
subsystem 130 and a data acquisition and communications subsystem
131. Since the user and service personnel interfaces are
conventional, for brevity no further discussion is provided.
[0036] For the purpose of controlling and monitoring the operation
of cogeneration system 100, the plurality of process control
sensors 126 and control outputs (I/O devices) 127 are individually
called out in Table 2 and shown in FIG. 1, by sensors
S.sub.0-S.sub.19, control points A.sub.1-A.sub.11, and protection
relays M.sub.1-M.sub.4, for the system at various points
therewithin. As illustrated in FIG. 2, the sensors and I/O devices
shown in Table 2 are interfaced with the controller 125 for control
of the micro-CHP system 100. Additionally, the controlled and
monitored parameter of sensors S.sub.0-S.sub.19 and control points
A.sub.1-A.sub.12 are also listed in Table 2, as well as the
preferred interface and type for each.
1TABLE 1 Component Mfg Part No Description Processor 1764LRP
MicroLogix 1500 Processor Base 176424BWA MicroLogix 1500 Base 24
I/O Analog Voltage 17691F4 Analog Voltage Input Module Input Module
Analog Voltage 17690F2 Analog Voltage Output Module Output Module
Right end cap 1769ECR Right end cap terminator terminator Cable
1761CBLPM02 Cable Thermocouple 1769-IT6 1769-IT6 Thermocouple Input
Module Input Module Real time clock 1764-RTC Real time clock Remote
access MICRORAD Remote access modem modem
[0037] The information obtained from sensors S.sub.1-S.sub.19 is
used by the controller 125 to provide detailed system control
through actuation and/or modulation of the various control points
A.sub.1-A.sub.11. Such controlled and monitored parameters include
pump speeds, gas flow rate, inlet and outlet temperatures and/or
pressures to, from, and within the expander 101, condenser 102,
evaporator 105, combustion chamber 114, desuperheater 106, and
throughout various points of the space heat, domestic hot water,
and generation loops 109, 112, and 122, respectively. Controlling
and monitoring at least three of the above mentioned parameters (as
will be discussed in later sections) enables setting the capacity
of the system 100 at any instant in time, thereby permitting load
following, using a variable capacity operation.
[0038] All of the pumps and fans 103, 110, 113, and 116, are
responsive to input signals from controller 125 via the their
associated control points, namely A.sub.10, A.sub.8, A.sub.7, and
A.sub.11, respectively. The controller 125 uses appropriate program
logic, as will be explained in a later section, to control the
turning on/off the fans, running and varying the speeds of the
pumps, and also opening and closing valves, such as for example,
by-pass valves 107, 121, shut off valves 118 and 120, and variable
flow gas valve 119, in response to predetermined conditions. Such
predetermined conditions include the demand for maximum heat or
power output and an electric grid outage. Valves 107, 118, 119,
120, and 121 are responsive to input signals from controller 125
via the their associated control points, namely A.sub.4, A.sub.1,
A.sub.6, A.sub.3, and A.sub.2, respectively.
[0039] The controller 125 also monitors a number of additional
safety controls and system protection devices, such as
relays/contactor 132 (FIG. 1) of the alternator to grid having
monitors M1-M4 listed in Table 2. Additionally, the controller 125
monitors and reports the electrical trips to the feed pump, an oil
pump, the hydronic pump, the blower, the gas valve, the expander
bypass valves, and any other electrically powered device in the
system. Furthermore, the controller may monitor and activate a
contactor of a crankcase heater for the power module via control
point A.sub.9 and switch the output of the generator 108 to the
power grid via control point A.sub.5.
[0040] In addition to controlling and monitoring system operations,
the controller 125 possesses functions to optimize the performance,
efficiency, and safety the system 100. The functions of the
controller 125 include automatic start up and shut down, modulation
control of hydronic supply temperature, monitoring and tripping on
safety and abnormal operating parameters, and electrical grid
connection interface. With reference also to FIGS. 3-4, these
functions of the controller 125 are explained in further
detail.
[0041] FIG. 3 is a flow chart of the operating logic of the system
100. In operation, the controller 125 in one embodiment is
programmed to operate the system in a quasi-steady state in
response to a need for heat that is keyed to a specified hydronic
supply temperature set point. This automatic operation is a heat
load following mode. In other embodiments, such a call for heat,
for example, may be from a thermostat, such as sensor S.sub.0,
which demands heat when the space temperature falls below a user
set-point, or according to an on-off timer, used for overnight
shutdown. In the latter case, a timer would enable the thermostat
to signal the controller 125 to initiate startup.
[0042] In the heat load following mode, the call for heat occurs
whenever the controller 125 determines that the temperature
differential between the actual hydronic supply temperature of the
DHW system 112, sensed via sensor S.sub.2, and a desired hydronic
supply temperature is greater the a predetermined value, such as,
for example 0.5 to 5.degree. Celsius.
2TABLE 2 Sensor-I/O Device Parameter PLC Interface Type S.sub.0
Space Temperature Digital Input Thermostat Or Analog Signal
Thermometer S.sub.1 Hydronic Condenser Inlet Temperature
Thermocouple Input Thermocouple S.sub.2 Hydronic Supply Temperature
Thermocouple Input Thermocouple S.sub.3 System Trip Data Logger
Error Signal S.sub.4 Expander Inlet Temperature Thermocouple Input
Thermocouple S.sub.5 Expander Inlet Pressure Analog Input Pressure
Transducer (0-500 Psi) S.sub.6 Feed Pump Inlet Temperature
Thermocouple Input Thermocouple S.sub.7 Feed Pump Inlet Pressure
Analog Input Pressure Transducer (0-200 Psi) S.sub.8 Power Module
Power Output Digital Input Amp/Watt Meter S.sub.9 Condenser
Internal Pressure Analog Input Pressure Transducer (0-200 Psi)
S.sub.10 or 10' Evaporator Coil Temperature Thermocouple Input
Thermocouple S.sub.11 Gas Flow Digital Input Flow Rate Meter
S.sub.12 Expander Outlet Temperature Thermocouple Input
Thermocouple S.sub.13 Feed Pump Set-point Digital Input Flow Rate
Meter S.sub.14 Heat Source High Limit Thermocouple Input
Thermocouple S.sub.15 Flame Detection Digital Input Electro Optical
S.sub.16 High Feed Level Digital Input Level Indicator S.sub.17 Low
Feed Level Digital Input Level Indicator S.sub.18 Outside
Temperature Digital Input Thermostat Or Analog Signal Thermometer
S.sub.19 Hydronic Fluid Flow Digital Input Flow Rate Meter A.sub.1
Main Gas Valve Relay Output Solenoid Valve Actuator A.sub.2
Expander Bypass Relay Output Solenoid Valve Actuator A.sub.3
Expander Shutoff Relay Output Solenoid Valve Actuator A.sub.4
Desuperheater Bypass Relay Output Solenoid Valve Actuator A.sub.5
Power Module Run/Stop Relay Output Power Module Contactor A.sub.6
Evaporator Bumer Firing Rate Analog Output Gas Proportioning Valve
Actuator A.sub.7 Hydronic Pump Run And Speed Relay Output Analog
Output Pump Variable Frequency Drive A.sub.8 Space Heat Fan
Run/Stop Relay Output Motor Contactor A.sub.9 Crank Case Heater
On/Off Relay Output Crank Case Heater Contactor A.sub.10 VFD - Feed
Pump Run And Speed Relay Output Analog Output Pump Variable
Frequency Drive A.sub.11 Forced Draft Fan Run/Stop Relay Output
Motor Contactor M.sub.1 Net Failure Digital Input Protection Relay
M.sub.2 Over/Under Voltage Digital Input Protection Relay M.sub.3
Over/Under Frequency Digital Input Protection Relay M.sub.4 Power
Module Internal Temperature Digital Input Digital Thermometer
[0043] It is to be appreciated that the hydronic pump 113 operates
continuously so there is always a flow through the DHW system 112
enabling the controller 125 to continuously monitor the actual
supply temperature and return temperatures, via sensors S.sub.2 and
S.sub.1, respectively. In other embodiments, the controller 125 may
use the internal temperature of the condenser 102 and hydronic flow
rate, via speed control of hydronic feed pump 113, to determine a
hydronic supply temperature using predetermined heat exchange
values and/or algorithms.
[0044] The desired hydronic supply temperature is set by the
controller 125 sensing the outdoor ambient temperature, via
thermostat S.sub.18, and correlating the sensed outdoor ambient
temperature to a desired hydronic supply temperature for the
boiler. The correlation between outdoor ambient temperature and
desired hydronic supply temperature may be, for example, according
to the illustrated linear relationship shown by FIG. 6. In this
illustrative embodiment, for example, on cold days, say -20.degree.
C. ambient, the hydronic set point is 75.degree. C., but on warm
days, when only a little heat is needed, i.e., 20.degree. C.
ambient, the set point is 25.degree. C., and at a 0.degree. C.
ambient the set point is 50.degree. C. However, for other
embodiments other scalar relationships between outdoor ambient
temperature and the desired hydronic supply temperature may be used
such as, for example, logarithmic, exponential, and other nonlinear
functions. To avoid the influence of sunshine on cold days, a
single measuring point on a north facing side of a building or home
should be used for thermostat S.sub.18.
[0045] Start-Up
[0046] In response to the call for heat in step 200, the controller
125 is programmed to modulate the thermal output of the evaporator
105 in order to have the actual hydronic supply temperature match
the desired hydronic set point for the given outdoor ambient
temperature. Accordingly, the controller 125 in step 202 determines
whether the evaporator 105 is already at its operating temperature
via thermocouple sensor S.sub.10 or S.sub.10, (FIG. 1). Assuming
the temperature of the evaporator 105 is below the minimum
operating temperature (e.g., cold start-up), the controller 125
verifies, and if necessary closes the expander shutoff valve 120
and opens the expander bypass valve 123 as shown in steps 204 and
206, respectively. Once verified, the controller 125 checks to see
if the burner is on via flame detector S.sub.15 and if not, purges
the combustion chamber 114 with force draft blower 116 and
activates the burner 104, as shown at blocks 208 and 210.
[0047] The burner 104 is activated by controller 125 opening the
gas valve 118 and metering its flow with flow rate valve 119, via
actuator A.sub.1, to nominally between 40 and 80% of full flow when
ignition is expected. An igniter (not shown) is turned off by a
timer and the gas valve is turned off when no flame is proven via
flame detector S.sub.15. The burner 104 will then typically be set
to come on to about 50% of its capacity to warm up system 100. If
desired, an off-the-shelf combustion controller can initiate burner
firing upon receiving an enabling signal from the controller 125.
If so arranged, the combustion controller will provide flame and
combustion air detection, light the burner, and provide high
temperature limit protection for the evaporator 105. The controller
also drives a variable speed draft fan to provide the approximately
correct air flow rate for the burner.
[0048] Next, the controller 125 checks to see if the evaporator
working fluid level is in the desired operating range, via level
sensors S.sub.16 and S.sub.17 in step 212 and if not, in step 214
will activate feed pump 103. It is to be appreciated that both
burner 104 firing and feed pump 103 flow may be controlled in part,
and conventionally by room temperature and its user determined
set-point, as well as outdoor temperature, via sensor S.sub.18.
Additionally, feed pump 103 comes on to a speed predetermined by
the controller 125 to coincide with the flow requirements
established by the initial burner firing rate and the design
response of the system 100. In particular, the controller 125 runs
feed pump 103 fast enough to keep the organic working fluid liquid
level between level low sensor S.sub.17 and level high sensor
S.sub.16. Accordingly, the feed pump is turned on by the controller
125 at the appropriate time to keep the evaporator filled, but not
over-filled with working fluid.
[0049] When the system is operating, superheated working fluid is
moving past sensor S.sub.4, which is able to provide a valid signal
to the controller 125 so the heat source or burner 104 firing rate
and feed pump 103 flow can be adjusted for both the safe operation
and needed output. However, when the system is just starting, the
controller 125 must be given some initialized state which can be
used as a safe operating condition until such time as working fluid
is flowing past temperature sensor S.sub.4.
[0050] It is desirable to have a minimum amount of working fluid
flow during startup, so that the fluid heats up as rapidly as
possible. However, some flow is needed to prevent local overheating
of the fluid in evaporator 105, and to provide the controller 125
with an indication that the burner 104 is indeed firing.
Accordingly, the burner gas rate is set to provide the longest
possible run time for the system, consistent with measured outdoor
temperature and rate of change of indoor temperature. Feed pump 103
operates to keep the evaporator 105 supplied with the working fluid
at the factory-preset value for temperature sensor S.sub.4. When
temperature sensor S.sub.4 gets to about 50% of the thermostat
set-point, feed pump 103 speed is increased until the temperature
reading in temperature sensor S.sub.4 reaches its set-point, at
which time the burner 104 modulates for constant values of at least
temperature sensor S.sub.4, and the feed pump speed is modulated to
maintain the desired hydronic supply temperature.
[0051] The controller will abort the start up sequence of steps
202-214, if burner 104 fails to heat the evaporator 105 or if any
of the parameters listed in Table 3 are exceeded. If the startup
sequence fails, the controller 125 will attempt to re-start again
for a maximum of three re-start attempts. If three re-start
attempts are completed with no successful re-start, the controller
125 will be locked out from re-starting until manually reset, via
trip reset 129 (FIG. 2). The re-start attempt counter will reset
after a successful start or a manual reset.
[0052] After the startup sequence is complete the system 100 will
go into run mode, wherein working fluid pressure is allowed to
build and feed pump 103, burner 104, and evaporator 105 are
controlled by controller 125 via two separate feed back loops.
[0053] At the appropriate time, the expander 101 is connected to
receive the now superheated vapors of the working fluid, wherein
the controller 125 in step 216 opens the cutoff or block valve 120
to expander 101 and after a short delay (i.e., 1 second), closes
the bypass valve 121. It is to be appreciated that liquid is
prevented from entering into the expander by maintaining the
evaporator's working fluid exit temperature at a fixed value for
all operating conditions. As mentioned previously, in one
embodiment, the evaporator exit temperature is set to operate at
about 154.degree. C. (310.degree. F.), which has been found to give
good overall system efficiency regardless of system load.
3TABLE 3 Parameter Threshold Fault Response Expander inlet
temperature 340.degree. F. (171.1.degree. C.) Full shut down
Expander inlet pressure 420 psia (29.5 Kg/cm) Full shut down Feed
pump inlet 200.degree. F. (93.3.degree. C.) Full shut down
temperature Feed pump inlet pressure 200 psia (14 Kg/cm) Full shut
down Protection relay trip Suitable trip values Power module shut
down only Power module temperature 300.degree. F. (148.9.degree.
C.) Power module shut switch down only Startup sequence failure 3
attempts Full shut down
[0054] Further protection to the scroll expander is provided during
start-up and while running by controlling the displacement of the
feed pump, which in one embodiment is by feed pump speed, wherein
the rate of change in feed pump speed is limited by an output ramp
in the controller logic which provides time for the evaporator PID
control to modulate the evaporator burner for steadier operation
and to prevent over temperature of the expander inlet. In this
control arrangement, a negative feed forward value will be added to
the feed pump speed control variable when ramping down to better
control temperature of the expander inlet.
[0055] The system 100 will warm up and quickly come to a
near-steady-state operating point for the feed pump flow setting.
After a short start-up delay (i.e., 1 second) in step 218 wherein
the controller verifies that the output of the generator is within
set parameters via sensor S.sub.8, the electrical output is then
connected to the grid in step 220 via contactor 132 responding to
control signal A.sub.5 from the controller 125. It is to be
appreciated that there is no need to control the generator output
as long as the generator output is less than the dwelling load.
Preferably, connection of the generator 108 to the grid loads the
alternator sufficiently to limit the speed to about 3150 RPM (50 hz
application).
[0056] However, should the controller 125 sense a power module
overspeed (e.g., loss of grid connection), it will quickly command
the opening of a bypass loop 123 around the expander 101.
Preferably, the sensor to detect expander overspeed is built into
the power module. Furthermore, the controller 125 can be programmed
with at least two options for dealing with the case when the
generator output exceeds the dwelling load. The controller 125 can
put the extra power on the grid or the extra power can be used for
a resistance heater at condenser 102.
[0057] Additionally, it is to be appreciated that the power module
108 will trip on over/under voltage, over/under frequency and loss
of mains conditions detected by the protection relay 128 (FIG. 2).
To prevent nuisance tripping of the heating system, only the power
module 108 will trip on voltage and frequency grid disturbances.
The controller 125 will log the fault and re-start the power module
108 after the over/under voltage or over/under frequency condition
has dissipated. If a loss of mains occurs, the entire system 100
will shut down with all latches/relays reset to start ready
condition. The controller 125 will then automatically re-start on
the restoration of power if heat is called for by the
thermostat/timer conditions.
[0058] In step 222, the controller checks to see if the call heat
has been satisfied via continuously reading sensor S.sub.0 and/or
S.sub.18. If the hydronic supply temperature is not at the desired
step point as checked in step 224, the controller 125 in step 226
reads the outdoor ambient temperature via sensor S.sub.18 and uses
a look-up table or algorithm to establish the desired set-point
according to the linear relationship illustrated by FIG. 6. The
controller 125 will then use the continuously updated desire
set-point to operate the evaporator 105 in a variable capacity mode
by modulating the gas valve 119 on the burner 104 to maintain the
actual hydronic supply temperature, sensed via sensor S.sub.2, at
the desired set-point.
[0059] In step 228, the controller 125 checks to see whether the
expander inlet temperature via sensor S.sub.4 is in a desired
operating range. If not, then in step 230 the controller 125
modulates the burner fuel flow rate (heat input rate) to provide
optimal thermodynamic performance and to prevent liquid from
entering the expander. Additionally, in this quasi-steady state the
controller 125 may control the hydronic pump 113 of the heating
system at a speed that maintains the pressure difference between
the supply and return headers of the condenser at a preselected
value for optimal thermodynamic performance of the hydronic heating
system.
[0060] With the controller subsystems 124 operating the system 100
in the above described heat load following mode, it is to be
appreciated that the system will operate for as many hours as
possible during the heating season. The controller subsystems 124
will run the system 100 just hard enough to maintain the hydronic
supply temperature at the correct value for the nominal heating
load. When the system 100 operates at less than the maximum supply
temperature, more power is generated than at the maximum
temperature, the controller 125 can automatically and passively
maximize the power which can be made and sold.
[0061] Further it is to be appreciated that in the above described
heat load following mode, the greater the temperature differential
error, the larger the thermal output response that will be
initiated by controller 125 in order to achieve the desired
hydronic supply temperature. As illustrated in FIG. 7, since the
thermal output of the system varies approximately linearly with the
mass flow of the system 100, the controller 125 controls its
thermal response by controlling both the displacement of the feed
pump 103, via a PID speed control, and the heat input to the
evaporator 105, via adjusting the gas flow rate to the burner 114
to maintain a 310.degree. F. (154.4.degree. C.) temperature into
the expander.
[0062] In particular, the controller 125 is programmed to increase
or decrease the mass flow rate in proportion to the system heating
capacity required to match the actual and the desired set point for
the hydronic supply temperature. For example, the desired set point
of the hydronic supply temperature is 50.degree. C. for a given
sensed outdoor ambient temperature of 0.degree. C., if while
operating in a quasi-steady state, the actual hydronic supply
temperature suddenly drops to 45.degree. C. (e.g., a door or window
of the space or building is open to the outdoors), the controller
125 will increase the percentage of system mass flow to meet this
heating demand. Should in this example after a predetermined time
period the temperature error between the actual and desired
hydronic supply temperatures decrease, the controller 125 will
further increase the percentage of system mass flow in order to
meet this heating demand. Accordingly, it is to be appreciated that
in normal operation the system 100 must operate at higher capacity
to attain the higher hydronic supply temperatures required by
colder weather and with the higher hydronic supply temperature the
capacity of the indoor radiators/convectors increases to handle the
added heat flow.
[0063] In run mode, the controller 125 monitors the operating
parameters shown in Table 1 and performs a full or partial shut
down if any of the parameters exceed preset thresholds. An
individual trip event on the automatic trip settings is defined as
a Level 1 trip. In the event of a Level 1 trip, the controller 125
will self reset and automatically attempt to re-start. If the
re-start fails, the controller 125 will attempt to re-start again
for a maximum of three re-start attempts. If the three re-start
attempts are completed with no successful re-start, this case is
defined as a Level 2 trip. In the event of a Level 2 trip, an alarm
notification message will be sent by controller 125, via the data
acquisition and communications subsystem 131 and the controller 125
will be locked out from re-starting until manually reset, via trip
reset 129 (FIG. 2). The re-start attempt counter will reset after a
successful start or a manual reset.
[0064] Shutdown
[0065] Normal shutdown of the micro-CHP will occur if the outdoor
thermostat signal indicates that the outside temperature has gone
above the temperature that corresponds to the lowest hydronic
supply temperature set-point and remains for 30 minutes. The system
will re-start when the thermostat signal indicates that the outside
temperature is below the normal startup temperature. The controller
125 will also perform the normal shutdown sequence when switched
off manually by the user or service personnel.
[0066] In the former case, when the system load falls below about
30 to about 40% of a full load and 30 minutes has expired, the
controller 125 is programmed to shutdown the system 100 and cease
making both heat and power to ensure economical use of the m-CHP
system. Since the hydronic pump is kept running at all times, even
at a low flow rate, the controller 125 continuously monitors the
error signal between the hydronic actual and set point values. When
this error is large enough, (i.e. the actual temperature is below
the set point by a preselected value) the controller starts the
system for another on-cycle. Should the controller also sense that
during operation of the system at the minimum system mass flow, the
actual supply temperature begins to exceed the set point, it is
programmed also to shutdown the system when this error exceeds a
predetermined value. In either of these conditions of heat load
following mode, or upon receiving a heat satisfied message from a
space thermostat, if not configured in the heat load following
mode, the controller 125 will follow a normal shutdown procedure
according to the program control logic illustrated by FIG. 4.
[0067] In step 300, normal shutdown begins with the controller 125
turning off burner 103. After a time delay in step 303 (i.e., 15
seconds), the controller 125 will shutdown (disconnect) the power
module 108, open the bypass valve 121 and close the shutoff valve
120 to the expander 101 in step 304. After another time delay
(i.e., 5 seconds) in step 306, the controller 125 stops the feed
pump 103 in step 308. After another time delay (i.e., 60 seconds)
in step 310, the controller 125 may slow the hydronic pump 113 to
its minimum speed, completing a normal shutdown of the system
100.
[0068] However, in step 300 the time delay may be conditioned on
available reservoir heat energy in heating system 112, wherein if
desired, the hydronic pump 113 may run on to deliver residual heat
in the hydronic system to the radiators until below a certain
set-point. In such a case, the controller 125 will initiate a
partial shutdown if the hydronic supply temperature exceeded the
set-point by 5.degree. C. for a period of 30 minutes when the
hydronic set-point is at the minimum setting. The evaporator 105,
feed pump 103, expander 101, and power module 108 will shutdown
while the hydronic pump will continues to run. When the hydronic
supply temperature error signal reaches -0.5 to 5.degree. C., the
system will re-start.
[0069] Data will be logged using the logging capability of the
controller 125. Preferably, a remote access modem will interface
with the controller 125 to download any system data. The modem may
be self-dialing and use either land line or cellular service. In
the event of a Level 2 trip, the controller may use the modem to
send an alarm notification message to service personnel. Data
points logged by controller 125 are shown in Table 4.
[0070] Evaporator Heat Control
[0071] In normal operation, no control of the evaporator heat rate
is required as the ideal heat input rate is set by a given mass
flow rate out of the feed pump. If the heat input rate is greater
than the ideal rate, then the evaporator outlet fluid will be more
superheated than desired. This will lead to increasing evaporator
pressure until the density at the expander inlet is sufficient to
provide a match between expander and feed pump mass flow rate.
Thus, the results of excessive heat input rate will be excessive
evaporator pressure and evaporator outlet superheat.
[0072] If there is too little evaporator heat input for a given
feed pump flow rate, evaporator pressure will be reduced, and some
liquid may be admitted to the expander 101. Liquid admission to the
expander 101 is not expected to result in damage to the expander
because of the increasing chamber volume at all times. However,
partial liquid admission is likely to result in reduced pressure
ratio across the expander 101 and, therefore, less thermodynamic
work by the expander. Accordingly, to obtain a better match between
the evaporator heat input rate and the feed pump flow rate over all
operating conditions, better control over the evaporator heat
output is desired.
[0073] The control concept involves sensing the level of superheat
at the evaporator outlet and maintaining this level in the range of
20.degree. F. to 30.degree. F. (-6.degree. C. to -1.degree. C.).
This will tend to minimize the evaporator pressure while
maintaining some margin from liquid entering the expander 101. The
evaporator pressure will then float until the density at the
expander inlet is such that the expander and feed pump flow rates
match.
4TABLE 4 Channel Parameter Signal Type 1 Hydronic condenser inlet
Thermocouple temperature 2 Hydronic condenser outlet Thermocouple
temperature 3 Hydronic fluid flow Analog 4 Expander inlet
temperature Thermocouple 5 Expander inlet pressure Analog 6 Feed
pump inlet temperature Thermocouple 7 Feed pump inlet pressure
Analog 8 Power module power output Analog 9 Evaporator inlet
temperature Thermocouple 10 Gas flow Analog 11 Expander outlet
temperature Thermocouple 12 Feed pump speed (drive frequency)
Analog 13 Protection relay trip Digital 14 Level 1 trip failure
Digital 15 Level 2 trip failure Digital (failed to restart)
[0074] The superheat exiting the evaporator 105 may be sensed by
either measuring the evaporator outlet temperature and saturation
temperature, measuring the evaporator outlet temperature and outlet
pressure, or sensing the saturation pressure. The first approach
requires measuring the temperatures of the working fluid in its
saturated state at the evaporator inlet with inlet temperature
sensor S.sub.10 (indirectly fired) or S.sub.10, (directly fired)
and also in its vapor state such as with expander inlet temperature
sensor S.sub.4. The difference between the temperatures is then the
superheat. For an improved superheat sensing, in addition to the
temperature, expander inlet pressure sensor S.sub.5 may be used to
sense the evaporator outlet pressure. The controller 101 may then
use a look-up table and/or calculation to determine the saturation
temperature from the pressure and temperature readings, wherein the
superheat is the difference between the measured temperature and
the computed saturation temperature. Furthermore, pressure sensor
S.sub.5 can also provide a safety feature to protect both the
evaporator 105 and the expander 101 from potentially harmful
overpressure.
[0075] The third approach is illustrated by FIG. 5, wherein the
evaporator outlet pressure is applied to one side of a diaphragm
500 of a superheat sensing device 501. The other side of the
diaphragm sees the same working fluid, but from a sensing bulb 502
at the same temperature as the evaporator outlet gas. The pressure
of this latter side is the saturation pressure corresponding to the
evaporator outlet temperature. If there is positive superheat, the
pressure from the sensing bulb 502 will exceed the evaporator
outlet pressure and the diaphragm 500 will compress a spring 504 by
an amount proportional to the degree of superheat. A potentiometer
(or other position sensor) 506 attached to the diaphragm 500 then
provides an electrical output to the controller 125 proportional to
the degree of superheat. The controller 125 can then use the output
sign from the superheat sensing device 501 to minimize the
evaporator pressure while maintaining some margin from liquid
entering the expander 101.
[0076] Although some aspects of the present invention are
identified herein as preferred or particularly advantageous, it is
contemplated that the present invention is not necessarily limited
to these preferred aspects of the invention. For example, an
alternative mode of providing heat following control may be as
follows. As before, this alternative heat following mode uses an
outdoor temperature sensor (i.e., sensor S.sub.18) to determine the
set point of the hydronic supply temperature. Controller 125 after
sensing the actual hydronic supply temperature (i.e., via sensor
S.sub.2), adjusts the burner firing rate, via the gas valve 118
and/or 119, via actuator A.sub.1, to bring the actual hydronic
supply temperature into line with the desired set point. If the
actual hydronic supply temperature is too low, the controller 125
will increase the firing rate, and if the temperature is too low,
the controller 125 will decrease the firing rate.
[0077] Further, in this alternative heat following mode, to
maintain the evaporator outlet temperature of the superheated
working fluid vapors within a desired operating parameter, such as,
for example 310.degree. F. (154.4.degree. C.), the controller 125
will adjust the feed pump flow rate and/or speed of the organic
working fluid past the burner in order to control the evaporator
exit temperature of the superheated vapor. If the exit temperature
of the superheated working fluid vapors from the evaporator is
above its desire operating parameter, the controller 125 will
increase the working fluid's flow rate. Should the exit temperature
of the superheated working fluid vapors from the evaporator be less
than its desired operating parameter, the controller 125 will
decrease the working fluid's flow rate.
[0078] Having described the invention in detail and by reference to
preferred embodiments thereof, it will be apparent that
modifications and variations are possible without departing from
the scope of the invention defined in the appended claims.
* * * * *