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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains to a low temperature (approximately 700.degree. F.
head temperature) Stirling engine driven by a solar powered apparatus.
2. Description of the Prior Art
The societal need for renewable, non-polluting sources of energy,
particularly electrical energy suitable for distribution by conventional
power lines, has become obvious in the face of increased environmental
concerns regarding global warming, acid rain and nuclear fuel disposal and
increased economic concerns regarding the high cost of energy production,
not to mention the long lead times required to build a conventional
large-scale power plant in the face of mounting regulatory obstacles.
While small-scale production of electricity by renewable means (such as
solar) is desirable in many respects, fundamental thermodynamic
constraints on the attainable energy conversion efficiency have tended to
make small-scale low-temperature heat source applications impractical.
However, high-temperature heat source applications have been difficult to
maintain, particularly if the thermal transfer fluid was a liquid metal
such as sodium or potassium which, while having the distinctive thermal
advantage of a low Prandtl number, is highly reactive with water and has a
tendency to leach alloying materials from the hot loop surfaces and
deposit them at the cold loop surfaces.
More particularly, in solar applications, the use of a high heater head
temperature (typically 1300.degree. F. in the past) to achieve high cycle
efficiency has necessitated a small aperture solar energy receiver (to
minimize aperture radiation losses) and, consequently, a collector with a
high concentration ratio and very accurate contour control; a
heat-pipe-type thermal transport subsystem to avoid hot spots on the
already high temperature head; the use of liquid metal heat pipe transfer
media (typically potassium-sodium mixtures) to achieve proper
heat-pipe-type operation at the temperature of interest; superalloy
materials in the engine-alternator heater head (due to concerns of the
creep strength of the material at the operating temperature, its yield
strength at both high and low temperatures, and its ability to resist high
and low cycle fatigue); and the use of helium rather than hydrogen as a
Stirling-cycle working fluid due to the high-temperature permeation of
hydrogen through the heater head into the heat pipe system. These
requirements adversely impacted the manufacturing cost and performance of
the system.
Moreover, a substantial drawback to solar-to-electrical energy conversion
has been the unavailability of sunlight during either nighttime or
inclement weather.
OBJECTS AND SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide a low-temperature
heat source solar-to-electric power conversion system which is practical
in view of fundamental thermodynamic constraints.
It is therefore a further object of this invention to provide a
low-temperature heat source solar-to-electric power conversion system
which avoids the complexity required by the use of liquid metal thermal
transfer fluids.
It is therefore a still further object of this invention to provide a
low-temperature heat source solar-to-electric power conversion system
which includes an auxiliary heat source for off-sun periods.
The apparatus of this invention is a low temperature (approximately
700.degree. F. head temperature) solar-to-electric power conversion system
which includes a solar collector, a cavity receiver, a pumped-loop thermal
transport system, a Stirling engine (which includes a low-temperature
Stirling thermodynamic cycle, a mechanical drive and an alternator), power
conditioning and controls, and a heat rejection coolant loop.
The solar collector receives incident solar radiation and focuses the
energy into the cavity receiver. The cavity receiver accepts the solar
radiation and heats a thermal transfer fluid in the thermal transport
system. The pumped-loop thermal transport subsystem transfers heats to the
Stirling cycle engine, which can be located away from the focal point of
the solar collector. The Stirling cycle and associated mechanical drive
and alternator convert the thermal energy into electrical energy. The
power conditioning and controls provide the electrical energy to the user
(either stand-alone or to the grid). The engine also rejects heat to a
coolant loop and radiator.
An optional auxiliary fossil or biomass burner may be added to the system
to heat the fluid in the thermal transport system to either supplement the
solar radiation or provide for operation at night. Additionally, heat
rejected from the engine through the coolant loop may provide hot water
for heating or for other thermal applications. The expected overall
performance of a system using a 700.degree. F. hot-side temperature
Stirling engine that rejects its heat at 160.degree. F. is 26.5% (useful
electrical energy to the user divided by energy to the receiver). This
estimate does not include possible energy recovery in the engine coolant
loop.
BRIEF DESCRIPTION OF THE DRAWINGS
Further objects and advantages of the invention will become apparent from
the following description and claims, and from the accompanying drawings,
wherein:
FIG. 1 is a block diagram of the system of the present invention.
FIG. 2 is a perspective view of a single stretched membrane dish of a solar
collector of the present invention.
FIG. 3 is a perspective view of the cavity receiver of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings in detail wherein like numerals refer to like
elements throughout the several views, FIG. 1 is a schematic of the
apparatus 10 of the present invention.
Apparatus 10 includes solar collector 12 which reflects and focuses solar
radiation to cavity receiver 14. Cavity receiver 14 receives thermal fluid
through primary thermal transport loop 16 from pump 18 and heats the
thermal fluid using the concentrated solar radiation from solar collector
12. Thermal fluid leaves cavity receiver 14 through primary thermal
transport loop 16 to auxiliary fossil fuel burner 20 which may be used
during inclement weather or at night. Thermal fluid, heated to
approximately 700.degree. F. in primary thermal transport loop 16 by
either cavity receiver 14 and/or auxiliary fossil fuel burner 20, is
provided to Stirling cycle engine 22 which provides mechanical work to
engine mechanical drive 24, alternator 26, and power conditioning and
controls 28 thereby providing electrical energy to either a stand-alone
user or an electrical utility power grid.
Meanwhile, in secondary loop 30, water or a similar liquid is pumped by
coolant pump 32 through Stirling cycle engine 22 to transport the rejected
waste heat away from Stirling cycle engine 22. This rejected waste heat
typically heats the water in secondary thermal loop 30 to approximately
160.degree. F. thereby allowing the secondary loop water to be used for
heating or similar purposes. Upon its return, if the secondary loop water
is not sufficiently cooled to attain the desired Carnot efficiency of the
Stirling cycle engine 22, cooling fan and radiator 33 are used to cool the
secondary loop water before it is pumped again to Stirling cycle engine
22.
The expected overall performance of apparatus 10 using a 700.degree. F.
hot-side temperature Stirling engine that rejects its heat is 26.5%
(useful electric energy to the user divided by energy to receiver 14).
This calculation does not account for possible energy recovery in the
engine coolant loop.
Referring to FIGS. 1 and 2, solar collector 12 intercepts the incident
solar radiation, concentrates it and directs it to the aperture 34 (see
FIG. 3) of cavity receiver 14. As solar collector 12 and cavity receiver
14 typically represent half of the cost of apparatus 10, improved cost
efficiencies in this area promise a reduced cost for the produced
electrical energy.
Preferably, collector 12 is comprised of a stretched membrane 36 which
offers simplicity, high performance, low cost, and light weight. Stretched
membrane 36 is a thin, flexible sheet of highly reflective material (e.g.,
silvered plastic film) stretched over circular hoop 38. Suction applied
behind membrane 36 causes membrane 36 to deflect into a spherical shape
suitable for use as a low-concentration-ratio point-focusing-type
collector.
For a given diameter of collector 12 (i.e., a given amount of intercepted
solar energy), increasing the amount of evacuation increases the skin
curvature, resulting in a shorter focal length for collector 12. A shorter
focal length simplifies the support 15 for cavity receiver 14. As the
curvature of membrane 36 is increased, the deviation of the surface from
the ideal paraboloid shape increases, and the maximum concentration ratio
of collector 12 decreases. An important advantage of a lower heater head
temperature is the ability of receiver 14 to interface efficiently with a
lower concentration ratio collector 12. Consequently, for a given focal
length, the diameter of collector 12 can be increased with only the mass
penalty associated with the additional surface area and not the mass
penalty associated with increased focal length. Alternatively, if the
reduced concentration ratio requirement is used to increase diameter and
decrease focal length simultaneously, it is possible to obtain an
increased collector size with no net weight penalty.
Pedestal 17 of collector 12 includes a clock-based sun tracking means to
maintain the proper orientation of collector 12 with regard to the sun
throughout the day and the seasons.
Referring to FIG. 3, the function of cavity receiver 14 is to accept the
concentrated solar radiation from solar collector 12 and convert it into
thermal energy for use by the remainder of apparatus 10. Cavity receiver
14 typically includes windowless aperture 34 into which solar radiation
enters. The solar radiation is converted to sensible thermal energy at
cavity walls 40 and transported by primary loop fluid within coils 42
concentric with cavity walls 40. Shell insulation layer 44 is placed
outwardly adjacent from coils 42 to reduce shell loss (i.e., thermal loss
from the hot inside surface of the receiver through the shell insulation
layer 44 to the surrounding ambient air).
The use of a lower head temperature (e.g., 700.degree. rather than
1400.degree. F.) reduces shell loss, cavity reradiation (the thermal loss
due to reradiation from the hot internal walls 40 through aperture 34,
which, to the first order, is proportional to the cavity temperature to
the fourth power) and transient start-up losses (i.e., the loss in thermal
energy required to bring receiver 14 to operating temperature, this loss
tends to be proportional to cavity temperature) of the cavity receiver 14.
Cavity convection (i.e., the free air convection through aperture 34 of
cavity receiver 14) tends to decrease with lower head temperature, due to
both the volume of convected air and the amount of energy lost per unit of
convected air. However, the use of a larger aperture 34 at low temperature
tends to increase the free convection rate and increase the energy loss.
Primary thermal transport loop 16 transfers the sensible energy generated
in cavity receiver 14 (and/or auxiliary fossil fuel burner 20) to the
engine heater head of Stirling cycle engine 22. Primary thermal transport
loop 16 could be a pumped liquid loop using a heat transfer fluid, molten
salt or liquid metal; a recirculating pressurized gas loop; or an
evaporation-condensation system such as a heat pipe (liquid return by
wick), a pool boiler (liquid return by gravity) or a reflux boiler (liquid
return by mechanical pump). For the instant implementation with a
700.degree. F. heater head temperature, a pumped liquid loop using
commercial heat transfer fluid (e.g., thermally stabilized silicone
polymer, such as Monsanto's Syltherm 800.RTM.) is preferred.
Stirling cycle engine 22 engine is a heat engine. Unlike internal
combustion engines, Stirling cycle engine 22 has a working fluid (usually
helium or hydrogen) contained within the engine. When the working fluid is
heated, the working fluid expands thereby pushing a piston. The piston
motion is then translated to output power either by engine mechanical
drive 24 which drives alternator 26 which, in turn, generates electrical
power. Alternately, in the case of a free-piston engine, the piston motion
is translated to expanding gas forces to a free-piston alternator
assembly. Once the piston has gone through an expansion stroke, the
working fluid is passed through three heat exchangers (not shown)--the
heater, regenerator, and cooler. The heater supplies heat energy to the
working fluid. The regenerator stores the heat energy supplied by the
heater. The cooler removes the heat from the working fluid and rejects the
excess to a secondary thermal loop 30. The cooled working fluid is then
compressed, passed through the regenerator to preheat the gas prior to
being heated, and expanded in the heater.
The cycle efficiency of Stirling cycle engine 22 can be considered as the
product of the two efficiencies as follows:
.eta..sub.cycle =.eta..sub.Carnot *.eta..sub.pneumatic
where .eta..sub.Carnot is the Carnot efficiency and .eta..sub.pneumatic is
the percent of the Carnot efficiency achieved in the cycle design.
The (ideal) Carnot efficiency is based only on the ratio of the engine
heater head temperature T.sub.h, to the engine cooler temperature T.sub.c,
wherein both T.sub.h and T.sub.c are expressed in absolute temperatures.
As the Carnot efficiency is considered a fundamental upper limit, it does
not depend upon the details of the engine design. The expression for the
Carnot efficiency is:
.eta..sub.Carnot =(T.sub.h -T.sub.c)/T.sub.h
For real engine systems, the Carnot efficiency can never be made equal to
one, but high values of Carnot efficiencies can be achieved by making the
heat rejection temperature low and the heater head temperature high.
On the other hand, the pneumatic efficiency depends on the specifics of the
cycle design, as well as the temperature ratio. Although always less than
1.0, the pneumatic efficiency of existing cycle designs tends to vary with
temperature ratio. It is low at low temperature ratios and it increases
with increasing temperature ratio to some maximum value beyond which it
decreases with further increases in temperature ratio. With past Stirling
design technology, the falloff in efficiencies below a temperature ratio
of 2.0 has been fairly steep.
The present apparatus 10 is particularly advantageous in that a cycle
efficiency of approximately 36% can be achieved with an engine operating
at a 700.degree. F. hot side and a 160.degree. F. heat reject temperature.
In the present apparatus 10, the "sink" for reject heat is the atmosphere,
and the extent to which the cooler temperature can be lowered is limited
by the ambient temperature and the size and cost of the cooling system
(i.e., cooling fan and radiator 33). As the cooler temperature approaches
the ambient air temperature, the size of the cooling system grows
exponentially, and cost considerations quickly prevent temperature
reductions beyond a certain point.
The remaining major system components--the engine mechanical drive 24, the
alternator 26, the power conditioning and controls system 28, and the
secondary thermal loop 30--are less affected by the heater head
temperature than are the front-end components discussed above. However,
advances in these systems aid in achieving a desirable economic cost of
the produced electricity as described herein.
Engine mechanical drive 24 for the Stirling cycle has two primary
objectives. First, it generates the volume variations in the cycle
expansion and compression spaces for proper cycle operation. Second, it
converts the pneumatic power generated in the cycle working fluid to
mechanical power and transmits it to the engine load.
Engine mechanical drive 24 may be either kinematic wherein the motions of
the pistons are rigidly constrained by a suitably configured crank system
or free-piston wherein the motions of the pistons are not rigidly
constrained and are achieved more subtly by the interaction of spring and
damper elements with the engine piston masses.
Each drive system has its advantages and disadvantages. The kinematic
engine is easier to control in the face of rapidly varying loads and
provides higher efficiency when shaft power rather than direct electrical
power is required. However, it normally uses piston ring seals and hence
requires periodic maintenance for seal replacement. The free-piston engine
typically uses noncontacting seals and bearings and therefore has the
potential for many tens of thousands of hours of operation without
maintenance. However, because of the difficulties inherent in the control
of apparatus 10, it is best used in applications characterized by a
relatively constant load. Free-piston drives are preferred for systems
intended to provide electric power to the utility grid (i.e., electric
output and small load variations). The kinematic drive is preferred when
the system is to be operated separately from the grid, primarily because
of the rapid load variations that typically occur in this mode of
operation. The kinematic drive disclosed in U.S. patent application Ser.
No. 694,370, filed May 1, 1991, is illustrative of engine mechanical drive
24 preferred in apparatus 10 for such applications.
Kinematic drives for small engine systems can be made very simple and
compact by using grease-packed bearings and dry-lubricated
(polytetrafluoroethylene, or Teflon.RTM.-type) wear pads and piston rings.
Power conditioning and controls system 28 is an important design
consideration in kinematic drives. However, the operational differences
between solar and fossil fuel energy sources can be used to significant
advantage to simplify the design of power conditioning and controls system
28. In fossil fuel systems, a major requirement of power conditioning and
controls system 28 is to reduce fuel consumption during periods of low
load. This typically requires the use of complex mean pressure or variable
engine stroke controls. In apparatus 10, the solar supply is freely
available independent of the load. Consequently, a major requirement of
power conditioning and controls system 28 is to prevent overspeed at low
load conditions. A mechanically simple parasitic power dissipation
control, such as a variable orifice valve between the engine working and
bounce space, can be used to manage power output.
When the solar insolation is insufficient to drive the load (either because
the insolation is too low or the load is too high), the engine shaft speed
is maintained by reducing the alternator field current and by allowing the
alternator voltage to drop. Many operational strategies can be postulated
for a system employing voltage drop to maintain the system power balance.
For example, specific loads can be dropped off-line at predetermined
voltage points or apparatus 10 can simply drop off-line below a
predetermined voltage, or suitable visual or audible alarms can be used to
alert the user to the need to reduce load.
To use apparatus 10, the user points solar collector 12 toward the sun (a
mechanical clock-type drive within pedestal 17 is used to maintain solar
collector at the proper orientation throughout the day); actuates pumps 18
and 32 of the primary and secondary thermal transport loops 16, 30; and
activates cooling fan and radiator 33. Optionally, the user may activate
auxiliary fossil fuel burner 20. The user then receives electrical power
from power conditioning and controls system 28 as generated by alternator
26 and hot water from secondary thermal transport loop 30.
Thus the several aforementioned objects and advantages are most effectively
attained. Although a single preferred embodiment of the invention has been
disclosed and described in detail herein, it should be understood that
this invention is in no sense limited thereby and its scope is to be
determined by that of the appended claims.
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