CA2736418A1

CA2736418A1 - A low temperature solar power system

Info

Publication number: CA2736418A1
Application number: CA2736418A
Authority: CA
Inventors: Nin G. Meng
Original assignee: Individual
Current assignee: Individual
Priority date: 2011-04-07
Filing date: 2011-04-07
Publication date: 2012-10-07

Abstract

This invention relates to a low temperature solar thermal power system, which combines a solar hot water system with an Organic Rankine cycle to generate electric power from sunlight. The low temperature solar power system typically includes multiple metal-glass tube solar collectors, a pump, a heat storage tank, and an organic Rankine cycle turbine/ generator set with propane as working fluid.

Description

A LOW TEMPERATURE SOLAR POWER SYSTEM
Inventor: Ning Meng (Edmonton, CA) Assignee: Ning Meng (Edmonton, CA) BACKGROUND OF THE INVENTION

[0001] The present invention relates to the methods and devices for a novel solar organic Rankine cycle power system by combining a solar hot water system with a propane organic Rankine cycle power system to produce solar electric power economically. In particular, the invention relates to the use of an improved working fluid in such organic Rankine cycle power system powered by low temperature solar thermal energy.

[0002] There is a continuing demand for clean renewable energy sources due to the depletion of the earth's supply of fossil fuels and concerns over the contribution to global warming from combustion of fossil fuels.

[0003] There is an enormous amount of solar energy provided by the sun to the surface of the earth that is available essentially free and without significant environmental impact. The amount of energy impinging at any particular area is a function of the geographic location, atmospheric conditions and season change. However, for many terrestrial locations, the solar energy impinges on the earth's surface does not require exploration, extraction of materials and refining.

Page 1 of 22 [0004] Some efforts to utilize this energy have been pursued, but with limited success. In one approach, photovoltaic ("PV") devices made of specialized silicon materials, able to directly convert sunlight into electricity. Though simple and clean, even after years of development, PV
devices remain quite expensive and cost prohibitive, resulting in long pay back periods. Also, PV
devices produce relatively low voltage direct current (DC), which has generally limited PV
devices to local use due to incompatible with and incapable of supporting the grid.

[0005] Solar thermal electric energy is another branch of solar energy. There are three kind solar thermal electric (STE) technologies: parabolic troughs, power towers, and dish/engine systems.

[0006] STE power is generated using heat from the sun. Solar collectors concentrate the energy of the sun to produce high temperature thermal energy between 400 C and 800 C, and this thermal energy is converted to electricity using conventional or advanced heat engines. Because their operation depends on the concentration of solar energy, STE technologies require high levels of direct-normal solar radiation and sun concentrating and tracking systems.

[0007] Solar thermal electric (STE) technologies use thermodynamic cycle process for converting heat into mechanic or electrical energy. Conventional solar thermal power plants create electricity by high temperature (over 400 C) energy, which requires complex devices for concentrating dilute solar energy to a concentrated energy at high temperatures. The cycle processes are operated in this case, for example, on the basis of the classic Rankine cycle with water as its working fluid. Its high boiling point however makes water unattractive for low efficiency and high cost, especially between 100 C and 200 C. For these reasons, only a small fraction, currently less than one percent, of electricity produced in the world exploits solar energy.

Page 2 of 22 [0008] Therefore, there is a need for economically viable approaches to produce electricity from solar energy. It is a good solution for cost reduction to produce electricity by using low temperature solar heat, without depending on the complex of sun tracking and concentrating system. The approaches described herein meet that need, which combine a solar hot water system with an improved organic Rankine cycle technology which is used for low temperature applications.

[0009] Solar hot water system is a very developed technology and is a very economical product.
There are thousands of manufactures in the world and over 20 billion dollars market every year.
Solar hot water system is economically available to compete with traditional hot water systems like nature gas system and mass production of solar collectors can make the combination of solar hot water system with organic Rankine cycle system economically viable. All solar hot water systems can produce hot water with a temperature about 60 C or somewhat lower.

[0010] There are basically three types of solar thermal collectors that are used for the solar hot water systems: flat-plate, evacuated-tube and concentrating.

[0011] Flat-Plate collectors comprise of an insulated, weatherproof box containing a dark absorber plate under one or more transparent or translucent covers. Water or heat conducting fluid passes through pipes located below the absorber plate. As the fluid flows through the pipes it is heated. This style of collector, although inferior in many ways to evacuated tube collectors, is still the most inexpensive type of collector for solar hot water systems.
At a temperature around 80 C, the thermal collecting efficiency is very low due to the large heat loss.

[0012] Evacuated Tube solar water heaters are made up of rows of parallel glass tubes. There are several types of evacuated tubes (sometimes also referred to as Solar Tubes).

Page 3 of 22 [0013] Type 1 (Glass-Glass) tubes consist of two glass tubes which are fused together at one end.
The inner tube is coated with a selective surface material that absorbs solar energy well but inhibits radiative heat loss. The air is withdrawn ("evacuated") from the space between the two glass tubes to form a vacuum, which eliminates conductive and convective heat loss. These tubes perform very well in overcast conditions and low temperature environments because the problem with loss of vacuum due to a broken seal is greatly minimized. Glass-glass solar tubes may be used in a number of different configurations, including direct flow, heat pipe, and U pipe.

[0014] Type 2 (Glass-glass - water flow path) tubes incorporate a water flow path into the tube itself. The problem with these tubes is that if a tube is ever damaged water will pour from the collector and the collector has to be shut-down and the tube replacement is required.

[0015] Type 3 (Metal-Glass) tubes consist of a single glass tube. Inside the tube is a flat or curved aluminum plate which is attached to a copper heat pipe or water flow pipe. The aluminum plate is generally coated with Tinox, or similar selective coating. At 80 C, only metal-glass tubes collectors are able to achieve a thermal efficiency high than 70%.
Insulation temperature of metal-glass evacuated tubes can reach 200 C from solar, average heat efficiency can still be more than 50%; even in an environment of below -50 C. This type of tubes is very efficient in solar heat collecting and is the best for high temperature application over 80 C.

[0016] For heat sources with such a low temperature, a wide diversity of technologies have been developed over recent years which make it possible to convert low temperature heat into mechanical or electrical energy with high degree of efficiency. A process known as organic Rankine cycle (ORC) with low boiling point organic working fluid stands out with better Page 4 of 22 efficiency. Various cycles for different applications between 100 C and 352 C
have been developed on the basis of the ORC with different working fluids.

[0017] The organic Rankine cycle (ORC) is a promising system for conversion of low and medium temperature heat to electricity. The ORC process works like a Rankine steam power plant but uses a low boiling point organic working fluid instead of water. A
certain challenge is the choice of the organic working fluid and of the particular design of the cycle. The process should have a high thermal efficiency and allow a high utilization of the available heat source.
Moreover, the working fluid should fulfill the safety criteria and be environmental friendly as well as low cost for a power plant.

[0018] Essentially, there are two known methods for using low temperature heat from thermal processes:

[0019] 1. For temperature of the heat source at least 140 C, the Kalina cycle system is preferably used. In a Kalina-process, the heat is taken away from the process medium by a saturated ammonia-water mixture, in which ammonia is desorbed. The ammonia expandes in an expansion turbine that is connected with a generator. Then the cold ammonia is reabsorbed in the ammonia-water mixture. The efficiency of the Kalinin process is approximately 18% (Brutto), a little higher than an ORC-process. Profitable and a significant wider range of temperatures of the working fluid are the simple reason to construct the plant. Disadvantageous are the problems with the construction material by using of the aggressive ammonia-water mixture which leads to lower the cycling time. Another disadvantage is the danger of possible emissions of the high-toxic and environmentally dangerous ammonia at possible leakages or disturbances.

Page 5 of 22 [0020] 2. For temperature of the heat source from 100 C to 352 C, an organic Rankine cycle system is preferably used. The Organic Rankine cycle (ORC) is a vapor power cycle with an organic fluid instead of water/steam as the working fluid. Functionally it resembles the steam cycle power plant: a pump increase the pressure of condensed liquid working fluid. This liquid is vaporized in an evaporator/boiler by extracting heat. The high pressure working fluid vapor expands in a turbine, producing power. The low pressure vapor leaving the turbine is condensed before being sent back to the pump to restart the cycle.

[0021] The efficiency of ORC is depend on the temperature difference between the temperature of condensation (temperature of the surrounding) and the reachable temperature of vaporization from 100 C to 352 C, and the types of working fluids also have a big impact on the efficiency of an organic Rankine cycle system. Many types of working fluids have been used in organic Rankine cycle turbine in the past, including refrigerants and various hydrocarbons. The efficiency of the ORC-process can reach nearly 10% at a temperature of 100 C
and nearly 20%
at a temperature of 200 C, are much low compare with Carrot cycle efficiency which is 18.7%
at a temperature of 100 C and nearly 35.9% at a temperature of 200 C.
Various refrigerants and various hydrocarbons as their working fluid have been utilized for various cycles for different applications between 100 C and 352 C. Chlorine containing fluids with high critical temperatures have been proposed in the past as ORC fluid. For example, R114, R113, RI 1, R141b and R123, have higher critical temperature than R245fa (154 C).

[0022] For low temperature solar power applications, U.S. Pat. No. 7,340,899 to Jeffrey et al discloses a low efficiency, low cost solar air motor generator system with the HCFC refrigerant wherein the solar energy collector is constructed from a plurality of heat exchanger of the kind used as evaporator in automobile air conditioners. Another U.S. Pat.
No.4,103,493 to James L
Page 6 of 22 discloses a low efficiency apparatus comprises in combination a direct boil solar collector which boils a refrigerant therein, a Ranking cycle engine for converting heat energy to kinetic energy with chlorine containing working fluids. Both of the patents could not choice the suitable working fluid for low temperature solar energy application with high efficiency.

[0023] Therefore, it is impossible to combine conventional ORC system with solar hot water system with that kind of working fluid efficiently. There is a need to improve the ORC working fluid and system for producing electricity from the temperature of the solar heat source between 60 C to 100 C with high efficiency economically. The approaches described herein meet that need, which combine a solar hot water system with an improved propane organic Rankine cycle technology to produce electricity with high efficiency. This invention makes it possible.

SUMMARY OF THE INVENTION

[0024] It is therefore an object of the present invention to overcome the problems of the prior art described above.

[0025] It is a further object of the present invention to provide a new and improved organic Rankine cycle working fluid for a low temperature solar hot water system.

[0026] It is a further object of the present invention to provide the high performance solar thermal collectors which can provide higher temperature for a new and improved organic Rankine cycle system.

[0027] Another object of the present invention is the provision of an organic Rankine cycle working fluid that can operate at higher pressure and lower temperature.

Page 7 of 22 [0028] Another object of the present invention is the provision of an organic Rankine cycle working fluid having a low critical temperature that is also high efficiency.

[0029] It has been discovered by this invention that some low critical temperature organic fluids has unique low temperature applications as a working fluid in an organic Rankine cycle system.
One example of a preferred fluid is propane, which has low boiling point (-41.9 C), low latent heat, and low critical temperature (96.8 C). The present invention provides propane as the organic Rankine cycle working fluid for a low temperature solar power system.

[0030] The present invention also proves that the metal-glass evacuated tube has high performance for a propane working fluid of organic Rankine cycle system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIG. 1 is a schematic illustration of the low temperature solar power system including the metal-glass evacuated tube solar thermal collector system and the propane ORC
power system.

[0032] FIG. 2 is a view of the metal-glass evacuated tubes solar collector utilized in this invention.

[0033] FIG. 3 is a detailed view of the metal-glass evacuated tube utilized in this invention.

[0034] FIG. 4 is a temperature-entropy (T-S) diagram for propane, a working fluid used in the low temperature solar power system.

[0035] FIG. 5 is a temperature-latent heat diagram for propane, as a working fluid used in the low temperature solar power system.

Page 8 of 22 [0036] FIG. 6 is a temperature-saturated pressure diagram for propane, as a working fluid used in the low temperature solar power system.

[0037] FIG. 7 is a condense temperature-ORC system efficiency diagram for propane, as a working fluid used in the low temperature solar power system.

[0038] FIG. 8 is an evaporation temperature-ORC system efficiency diagram for propane, as a working fluid used in the low temperature solar power system.

DETAILED DESCRIPTION OF THE INVENTION

[0039] A low temperature solar power system is invented by combining a high efficiency low cost solar hot water system with a high efficiency ORC system. More specifically, instead of custom components and devices that incorporate exotic materials, this invention combines high efficiency and less expensive solar metal-glass evacuated tube collectors with a high efficiency ORC system to make the solar power system economically viable.

[0040] Theoretically, Different types of solar collectors have a significant impact on the efficiency of an organic Rankine cycle system, and primarily on the operating temperatures and pressures of the cycle. In the past, many types of solar collectors have been used to collect solar thermal energy efficiently for the solar hot water system under 60 C. At a temperature over 60 C, which is the need for ORC system, only metal-glass tubes are able to achieve a thermal efficiency high than 70% and then mach the need of organic Rankine cycle.
Insulation temperature of metal-glass evacuated tubes can reach 200 C from solar, and average heat efficiency can still be more than 50%; even in an environment of below -50 C.

Page 9 of 22 [0041] Types of working fluids also have a big impact on the efficiency of an organic Rankine cycle system for the various thermodynamic cycles in which the turbine operates. Many types of working fluids have been used in organic Rankine cycle turbine in the past, including refrigerants and various hydrocarbons. The selection of the working fluid will depend on the range of solar heat temperature and heat sink temperature of a condenser in a closed loop of ORC system. In the present invention, the propane is selected as the working fluid to be used in the closed loop of ORC system, with an solar heat temperature range of 60 -96.8 C, and a heat sink temperature of a condenser ranging from -20 to 40 C, relating to propane low boiling point temperature -41.9 C and low critical temperature 96.8 C.

[0042] The selection of the working fluid is a key importance in low temperature Rankine Cycles. Because of the low temperature, heat transfer inefficiency is highly prejudicial. The inefficiency depends very strongly on the thermodynamic characteristics of the fluid and on the operating conditions. In order to recover low-grade solar heat, the working fluid must have a lower boiling temperature. Since the purpose of the ORC focuses on the low grade solar heat, a superheated approach like the traditional Rankine cycle is not appropriate. In the case of dry fluids, generally a regenerator should be used. A fluid with a low latent heat will have high efficiency as it ejects less heat energy to the condenser of the ORC system and thus reduces the required flow rate, the size of the facility, and the pump consumption. The freezing point of the selected working fluid should be lower than the lowest temperature in the cycle and also has a low environmental impact.

[0043] Generally, the organic Rankine cycle (ORC) is a very developed process for conversion low and medium temperature heat to electricity from a temperature range of 80 C -352 C. But Page 10 of 22 there is no ORC system for conversion low temperature heat to electricity from a temperature range of 60 C - 80 T.

[0044] The present invention uses propane as its working fluid in an ORC for low temperature solar power due to its low boiling point (-41.9 C) and small latent heat characteristic, as the results, is able to achieve a high efficiency even at a very low temperature.
Various refrigerants and various hydrocarbons as their working fluid have been utilized for various cycles for different applications between 100 C and 200 C, wherein R245fa, with lower critical temperature (154 C) is used for lower temperature applications. The properties Comparing of saturated pressure between Propane with R245fa is showed in table. At low temperature, propane has much high saturated pressure than R245fa. For example, at the temperature 80 C which is the operating temperature of this invention, saturated pressure of propane is 31.3 Bar, much high than R245fa (7.8 bar). This is the reason that propane can make more power than R245fa at this temperature, and consequently the efficiency of propane ORC is much high than R245fa ORC.
Present invention addresses the working fluid with low critical temperature, and high critical pressure at the operating temperature area of ORC.

A comparison of saturated pressure of Propane with R245fa.
Temperature Propane R245fa C Bar Bar 60 21.2 4.6 80 31.3 7.8 100 42.5 12.4 120 42.5 19.2 140 42.5 28.2 160 42.5 36.4 Page 11 of 22 [0045] FIG. 1 shows a schematic of the low temperature solar power system 10, which generally includes a solar hot water system 20 and an organic Rankine cycle system 30 with propane as the working fluid. A solar hot water storage tank 28 is used to provide thermal energy to propane organic Rankine cycle system 30 up to 24 hours a day. The power generated by generator 38 may be used in various applications, including, but not limited to: powering commercial and residential buildings.

[0046] In operation, the heat transfer fluid is pumped through the pump 22 to solar collectors 21 from the storage tank 28. The heat transfer fluid flows through solar collectors 2lwhere it is heated by the solar energy. Solar collectors 21 are capable of withstanding temperatures of at least approximately 250 C.

[0047] After the heat transfer fluid is heated in the solar collectors 21 to the desired temperature about 90 C, the heat transfer fluid flows into hot thermal storage tank 28.
The heat energy is then stored in the hot thermal storage tank 28 until it is needed by propane ORC system 30 to produce electricity. Hot thermal storage tank 28 allows for power production during cloudiness or darkness.

[0048] The heat transfer fluid using for this solar thermal system can be any fluid that has the capability to transfer heat and thermally maintain the heat in the fluid, such as silicon oil, water, antifreeze mixture, or propane directly. In an exemplary embodiment, glycol antifreeze mixture is used as the heat transfer fluid through solar heating system 20. The glycol antifreeze mixture used to transfer heat from solar collector system 20 to propane ORC system 30 is capable of being heated to a temperature of at least approximately 100 C.

Page 12 of 22 [0049] When electricity generation is needed, the propane is pumped through the thermal storage tank 28 to high pressure, thus induce a phase change in the heat exchanger from a liquid phase to a high pressure gas phase. The turbine 31 is rotated as the expansion of the heated high pressure propane gas. The electrical generator 38 is coupled to the turbine so that rotation of the turbine 31 causes rotation of the generator 38 to make electricity. The propane ORC
system 30 also includes a condenser 34 to induce a phase change in the condenser from a gas phase to a liquid phase. Condenser 34 may reject the heat into water, which is sent to a cooling tower to release the heat to the atmosphere. Alternatively, the heat rejection may also be accomplished by directly air cooling.

[0050] As shown in FIG. 1, solar hot water system 20 generally includes circulation pipe 23, solar collectors 21, pump 22, and hot storage tank 28. Circulation pipe 23 transports the heat transfer fluid through the solar collectors heating system 20 and carries the heat transfer fluid from the storage tank 28 to the solar collectors 21. A secondary line 26 carries the heat transfer fluid from the solar collector 21 to the hot storage tank 28 and back to solar collectors 21 in a closed loop of the solar system.

[0051] FIG. 2 shows the metal-glass evacuated tube solar collector 21 comprising an insulation layer and case 24, an in/out let 25, multiple high efficiency metal-glass evacuated tubes 27 with sealing rubbers, tube caps 29.

[0052] FIG. 3 is a detailed view of the metal-glass evacuated tube 27 connecting to the solar.
The metal-glass evacuated tubes 27 consists of a single glass evacuated tube 272. Inside the tube is a flat or curved aluminum plate 273 which is attached to a copper heat pipe 274. The aluminum plate 273 is generally coated with a selective surface material that absorbs solar Page 13 of 22 energy well but inhibits radiative heat loss. The air is withdrawn ("evacuated") from the space of the glass tubes to form a vacuum, which eliminates conductive and convective heat loss. These tubes 27 perform very well in overcast conditions as well as low temperatures.
These types of tubes 27 are very efficient, and are best suited to an ORC system.

[0053] As shown in FIG. 1. The propane ORC system 30 generally includes a cycle pump 39; a storage tank 28 with coil heat exchanger 32, a turbine 31, a turbine generator 38, a condenser 36, and circulation pipes 33. The working fluid propane is pumped and circulated in the closed loop 33 of ORC system. In addition, generator 38 and turbine 31 are connected on a shaft 35. In an exemplary embodiment, a turbine 31 is a propane organic Rankine cycle power conversion system.

[0054] As shown in Fig. 1, in operation, when electricity generation is needed, the propane is pumped by the pump 39 to and through the storage tank 28. The pump 39 is coupled to the heat exchanger 32 in the storage tank 28 in fluid communication with the turbine 31 to induce a phase change in the heat exchanger 32 from a liquid phase to a high pressure gas phase. The turbine 31 is coupled to the outflow heated high pressure propane gas from the heat exchanger 32 in the storage tank 28 and is rotated by the heated high pressure propane gas. The electrical generator 38 is coupled to the turbine 31 so that rotation of the turbine 31 causes rotation of the generator 38. The propane ORC system 30 also includes a condenser 34 in fluid communication with the turbine 31 to induce a phase change in the condenser 34 from a gas phase to a liquid phase.
Condenser 34 may reject the heat into water, which is sent to a cooling tower to release the heat to the atmosphere. Alternatively, the heat rejection may also be accomplished by directly air cooling. The output of the condenser 34 is coupled to the gas trap valve 37.

Page 14 of 22 [0055] In this exemplary embodiment, the thermal energy is exchanged from the storage tank 28 to the propane liquid of ORC system 30 in the coil heat exchanger 32, and the propane is heated to a temperature of approximately 80 C thus change its phase from a liquid to a high pressure gas with a pressure of approximately 31.31 Bar, as it leaves the heat exchanger 28 and flows through the high pressure line 33 to the turbine 31. At turbine 31, the high pressure propane gas is allowed to expand and release energy, reducing the temperature of the propane gas to approximately 40 C, and pressure approximately 13.69 Bar with dry condition.
The energy released during the expansion process in turbine 31 is sufficient to turn the generator 38 on shaft 35. Generator 38 uses the mechanical energy from the turbine 31 to generate electricity. The propane is then transported from turbine 31 to condenser 34 through temperature line 36. The temperature of the propane gas drops to approximately 30 C in the condenser 34.

[0056] FIG. 4 is a temperature-entropy (T-S) diagram for propane, a working fluid used in the propane ORC system 30. The propane liquid is pumped from state point I to point 2 increasing the pressure, and preheating to approximately 80 from state point 2 to point 3, thus evaporating to approximately 31.31 bars from state point 3 to point 4 in the thermal storage tank 28 by solar heat energy. At turbine 31, the high pressure propane gas is allowed to expand and release heat energy to produce power, reducing the temperature of the propane gas to approximately 40 C
from state point 4 to point 5 to the pressure approximately 13.69 Bar at dry condition. The propane gas desuperheating to approximately 30 C in a condenser or regenerator from state point 5 to point 6, thus condensing for rejecting the latent heat from state point 6 to point 1, then the propane changes its gas phase back to liquid phase. In this exemplary embodiment, the efficiency of the propane ORC system 30 is approximately 16.4%.

Page 15 of 22 [0057] Conventionally, a regenerator is used to transfer some of the superheat to the liquid in the boiler before boiling occurs. However, this type of working fluid is inefficient because it involves a vapor/liquid heat exchanger requiring large heat transfer surface, thus resulting in a costly and complex system. In this exemplary embodiment, the efficiency of the propane ORC
system is approximately 15.6% without regenerator, close to the efficiency of 16.4%. Thus, in order to reduce system complexity, a regenerator is eliminated from the propane ORC system.

[0058] FIG. 5 is a plot of propane latent heat-temperature diagram, illustrating thermal characteristic of a propane ORC system 30. For example, at the condense temperature 30 C, the propane latent heat is only 14.2 KJ/mol, less than half comparing with steam latent heat (40.68 KJ/mol) of water. The propane latent heat decreases at critical temperature 96.8 C dramatically;
consequently the propane ORC system 30 will have more high efficiency than water Rankine cycle for rejecting less energy in condenser 34.

[0059] FIG. 6 is a propane saturated pressure-temperature diagram. Comparing with water boiling temperature (100 C) at 1 atmospheric pressure, propane not only has a very low boiling temperature (- 41.9 C) at the pressure of 1 atmosphere, but also has a low critical temperature (96.8 C) at 41.9 Bar ; suggesting propane ORC system 30 can have a very low operating temperature ranging from -41.9 C to 96.8 C. For the exemplary embodiment, at the evaporating temperature 80 C, the propane saturated pressure is 31.31 bar, much higher than that of other ORC systems. This is the reason propane is selected for low temperature solar power system 10.

[0060] FIG. 7 is variations of propane ORC 30 efficiency as a function of condense temperature.
The cycle efficiency of the propane ORC system 30 depends on the temperature of rejection in Page 16 of 22 the condenser 34. The efficiency is 16.4% at the normal condense temperature (30 C); while at the cold condense temperature (-20 C), the efficiency will increase to 26.6%.
It is showed that when the heat rejection is accomplished by direct air cooling in the propane ORC 30, more high efficiency is able to achieve at cold areas.

[0061] FIG. 8 is a plot of efficiency of propane ORC 30 versus evaporation temperature. The cycle efficiency of the propane ORC system 30 depends on the temperature of evaporation in the heat storage tank 28. At the normal condense temperature (30 C); the efficiency of the propane ORC 30 is 21.7% at the critical temperature 96.8 C, and decreases to 11.5% at the evaporation temperature 60 C. This confirms that the metal-glass collector 21 is best suited for a propane ORC system.

[0062] Due to propane's low boiling point (-41.9 C) and low critical temperature (96.8 C) characteristics, propane ORC system 30 is able to achieve a high efficiency even in winter with the advantages of these thermodynamic properties. This feature of propane provides a potential to keep a stable high efficiency of propane ORC system 30 in both summer and winter. In summer, the metal-glass evacuated solar collector 21 can achieve more high temperature efficiently, the evaporation temperature could reach its critical temperature 96.8 C, coupling with the normal condense temperature 30 C. The thermal efficiency of the solar collector system 20 is about 70%, and propane ORC system 30 efficiency is 21.7%, consequently, the total efficiency of the low temperature solar power system 10 is 15%. In winter, the metal-glass evacuated solar collector 21 can collect solar thermal energy more efficiently in low temperature range; the evaporation temperature could be 80 C, coupling with the cold air temperature 0 C.
The efficiency of the solar collector system 20 is about 70%, and propane ORC
system 30 efficiency is 22.2%, consequently, the total efficiency of the low temperature solar power system Page 17 of 22 is 15.54%. This achievable efficiency at low cost is high enough to compete with PV or STE
solar power system.

[0063] Low temperature solar power system 10 can range in size from 5 to 250 kw; multiple propane ORC systems can be used to form a power plant of any size over than 250 kw. The power generated by a low temperature solar power system 10 may be used in various applications, including, but not limited to: powering commercial and residential buildings.

[0064] In an exemplary embodiment, a low temperature solar power system 10 generates approximately 5 KW of electrical energy for a residential house, with an efficiency of approximately 15%.

[0065] Although the present invention has been described with reference to a preferred embodiment, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.

Page 18 of 22

Claims

1. A method and apparatus of converting solar thermal energy to electrical power by using a low temperature solar organic Rankine cycle (ORC) system in which a working fluid circulates comprising (I) an ORC system that includes an evaporator, a turbine, a generator, a condenser, a gas trap value and a cycle pump; the generator and the turbine are connected on a shaft;

(II) a solar hot water system that includes a thermal storage tank, a pump, and plurality of metal-glass evacuated tube solar collectors.

2. The method and apparatus of claim 1, wherein the ORC system includes organic working fluid propane, which is heated and evaporated in the evaporator from a liquid phase to a gas phase, then the gas carries heat provided by the solar hot water system to the turbine and drives the turbine then is condensed back to the liquid phase in the condenser.

3. The method and apparatus of claim 1, wherein the metal-glass evacuated tube solar collectors gather and convert solar energy to heat energy, and then transfer the heat energy to the heat transfer fluid that circulates in the solar hot water system and keeps its heat in the thermal storage tank for the ORC system.

4. The method and apparatus of claim 1, wherein the solar collectors of the solar hot water system further comprising insulation layer and a box, a in/out let, multiple high efficiency metal-glass evacuated tubes with sealing rubbers, tube caps.

5. The method and apparatus of claim 1, wherein the metal-glass evacuated tube further consists of a single glass evacuated tube. Inside the tube is a flat or curved aluminum plate which is attached to a copper heat pipe. The aluminum plate is generally coated with a selective surface material that absorbs solar energy well but inhibits radiative heat loss. The air is withdrawn ("evacuated") from the space of the glass tubes to form a vacuum, which eliminates conductive and convective heat loss.

6. The method and apparatus of claim 1, wherein the ORC system further comprise working fluid propane being pumped and circulated in the closed loop of ORC
system.

7. The method and apparatus of claim 1, wherein the condenser receives the organic Rankine cycle working fluid gas from the turbine and cools it.

8. The method and apparatus of claim 1, wherein a propane gas trap only allows liquid propane to pass and keeps propane gas from passing it.

9. The method and apparatus of claim 1, wherein the cycle pump can increase the liquid propane pressure.

10. The method and apparatus of claim 1, wherein the solar collectors heat a heat transfer fluid, such as water, an anti-freezer mixture or silicon oil, to a temperature of at least about 90 °C.

11. The method and apparatus of claim 1, wherein the evaporator in the storage tank operates at temperature about 60 -100 °C.

12. The method and apparatus of claims 1 and 10, wherein the metal-glass tube solar collector hot water system heats the heat transfer fluid to a temperature of at least about 80-100 °C.

13. The method and apparatus of claim 1, wherein the low temperature ORC
system further comprise a thermal storage tank with the evaporator, where the thermal energy is transferred from the thermal storage tank to the ORC.

14. The method and apparatus of claim 1, wherein the low temperature solar ORC
can keep a stable high efficiency about 15% in both summer and winter.

15. The method and apparatus of claim 1 can provide power ranging from 5 to 250 kW;
multiple propane ORC systems can also be provided to form a power plant of any size over 250 kW. The power generated by low temperature solar power system may be used in various applications, including, but not limited to: powering commercial and residential buildings.

16. The method and apparatus of claim 1, wherein the turbine can be a turbine, or any kind of expansion machine using propane as working liquid.