Patent Application
20090183853
Cooling and heating of buildings, especially residential houses, consume a huge amount of energy worldwide. Because solar energy is directly accessible to a large percentage of the residential houses, using solar energy to cool and heat the houses is highly desirable. For many decades, concepts of cooling and heating systems based on solar energy have been proposed. However, none of those designs has been widely used.
Several combined solar cooling and heating systems have been disclosed that comprise a solar collector, typically on the roof, to generate hot water (or other liquid) for the storage of thermal energy; then use the stored thermal energy for heating, or for cooling using the heat-pump principle. For example, Worthington in U.S. Pat. No. 4,128,124 (1978); Adcock in U.S. Pat. No. 4,129,177 (1978); Ruder in U.S. Pat. No. 4,153,104; Hiser in U.S. Pat. No. 4,173,994; Yukimachi et al. in U.S. Pat. No. 4,269,263 (1979); Wilson in U.S. Pat. No. 4,674,476 (1987); and Tracy in U.S. Pat. No. 5,941,238 (1999). Those systems are complicated and bulky, having low efficiency because of thermal-storage loss and indirect heating and cooling, requiring dedicated design for new houses substantially different from ordinary houses, and almost impossible to retrofit into existing houses. Those systems have not been widely used in practice.
A simple idea for solar heating of houses has been practiced using a thermal storage medium, as summarized in E. Marzia's “The Passive Solar Energy Book” (Rodale Press, Emmaus, Pa. 1979); and in D. Chiras's “The Solar House” (Chelsea Green Publishing Company, White River Junction, VM 2002). Typically, masonry thermal storage walls (made of adobe, bricks or concrete) are used as the thermal storage medium. In a sunny winter day, direct sunlight is allowed to come through windows to heat up the masonry wall. The heat stored in the wall is then gradually released after sunset. The temperature of the house or room can be maintained within a comfortable range day and night. However, a masonry wall with sufficient thermal mass is very bulky and heavy which requires a large space and a strong foundation. Therefore, it is not widely used.
Solar cooling has a similar thermal-storage problem. By directly using the solar energy to drive an air-conditioning unit for cooling the room, it does not provide a relatively constant temperature throughout the entire day and night. Right after sunset, when the environment is still very hot, the cooling effect disappears. And the maximum cooling effect from direct sunlight is at noon time, which is not the hottest time of the day (the hottest time in a day is about 3-5 pm). This problem can be resolved by using a thermal storage medium to store the cooling power generated by solar energy with 4 to 8 hours of delay or inertia. Because generally speaking, the stronger the sunlight in the day time, the more cooling power is required, it can be self-sufficient from day to day.
However, to store cooling power, a proper design of the thermal storage system must be provided. Masonry walls are not useful as a thermal storage for cooling because of the difficulty to cool it down. It is well known that water is a much better thermal storage medium, as shown in Table 1. To achieve the same thermal mass, the weight of water is only one tenth of that for brick or concrete. Moreover, water has great advantage as storage medium of cooling because of natural convection: once a source of cooling is accessed to the top of a water container, cooling spreads quickly to the entire volume. Although it is well known that water is much more advantageous even for solar heating, the technology and design of reliable and affordable water storage systems does not yet exist. In the field of passive solar heating, most applications utilizing water as thermal storage medium have been using either stacked 55-gallon drums or freestanding metal and plastic cylinders; see the above cited books of E. Marzia and D. D. Chiras. Those systems are cumbersome. Therefore, to date, water storage systems for passive solar heating are not publicly accepted. For solar cooling, to date, water as a cooling storage medium has not been explored.
The current invention discloses a novel apparatus for solar-powered cooling and heating, which comprises one or more water containers of special design to form a structured water wall, placed near a south-faced window in a room as the thermal mass for solar cooling and heating. Using proper design parameters, it could have a thermal inertial delay time of 4 to 8 hours. A solar photovoltaic panel is installed on an awning above the south-faced window. The output of the solar photovoltaic panel is connected directly to a DC motor to drive a compressor of a vapor-compression refrigeration unit. For the principles of vapor-compression refrigeration, see for example W. P. Jones “Air Conditioning Engineering” (Edward Arnold, London 1994). The heat generated by the compressed refrigerant (for example, R22) is dissipated to the surrounding air, and the liquefied refrigerant is then letting into a copper coil through an expansion valve. The copper coil is located near the top of a water container to cool down the water. In a sunny summer day, the electricity generated by the sunlight drives the compressor and cools the water. By natural convection, the entire water wall is being cooled. The temperature of the water wall reaches minimum in the late afternoon, typically 3 pm to 6 pm, which matches the hottest time of the day. Because the structured water wall is located in the room, it cools down the entire room. After sunset, the temperature of the structured water wall gradually increases. Because the environment temperature is also decreasing, the warming up of the water wall is slowing down. The cooling effect persists throughout the night. Such a system has a self-adjusting effect according to the weather: The stronger the sunlight in the day time, the more cooling is generated by the water wall. On the other hand, sunlight could fall on the water wall directly through the window. However, in the summer, the sunlight is blocked by the awnings (which support the solar photovoltaic panels) from coming directly through the window, and the insulating shutters are closed. No heating effect is caused by direct sunlight. In the day time of the winter, by opening the insulating shutters, full sunlight comes through the window and heats up the water wall. In the night, by closing the shutters, the heat is releasing slowly to keep the room warm. In cold and cloudy days, an auxiliary heating system is required. However, substantial savings of the heating cost can be achieved. The said solar-energized cooling-and-heating apparatus can be installed into new houses of standard design, can be retrofit into existing houses, can be decorated as a good-looking furniture (screen, bookshelf, entertainment system, picture-displaying wall, etc.) and can be mass-produced with low cost. The operation of the apparatus is simple and virtually maintenance free.
In front of the structured water wall 105, insulating shutters 116 are installed. The shutters are preferably made of plastic foam, about one inch thick, and covered by aluminum foil or colored white. During the summer, the shutters are closed to become a continuous thermal insulating panel to the water wall, to avoid thermal loss through the window. During the day time of winter, the shutters are opened to an angle approximately equal to the inclination angle of the sunlight. The surface of the structured water wall is colored black to ensure good absorption of heat. After sunset, the shutters are closed. This can be controlled by a timing device or a light-sensing device. The back side of the water wall is protected by a panel 117, which is decorated as a screen in this case. Also in the room, a bed 118 is placed nearby. The panel 117 can also be decorated as an entertainment center or a bookcase as appropriate.
To facilitate mass production, the preferred design of the water container comprises two identical halves, as shown in
The operation of the apparatus is as follows. In a sunny day of summer, the solar photovoltaic panels 110 receive plenty of sunlight to generate electricity. Through the DC motor 404, it directly drives the compressor 405 of the refrigerating unit. The refrigerant, for example R22, is compressed, and the heat generated is dissipated to the environment by the fan 406. The refrigerant is then liquefied. It passes through an expansion valve 409 to become vapor, and then flows through pipe 410 to the heat exchange coil 411 near the top of the water wall 105. If there is a single water container, the cooling effect spreads quickly to the entire volume by convection. For systems having two or more water containers, see
A typical temperature profile in the summer is shown in
A typical temperature profile in the winter is shown in
Following is a thermodynamic calculation for estimating the proper sizes of the elements to achieve maximum comfort.
First, a moderately sized structured water wall can provide extraordinary comfort for a typical room of 12 ft wide and 16 ft long and 8 ft high in a residential building. Suppose that two sides of the room have walls towards the exterior of the house. In SI units, the area of the walls is approximately 20 m2 and the area of the windows is 4 m2. For modern houses, the typical U-value of the insulated walls is 0.3 W/° C.m2 (Watts per degree Celsius per square meter), and the typical U value of the window is 1.4 W/° C.m2. The total rate of heat loss is 11.6 W/° C. Suppose that the average external temperature is 30° C., and the average temperature of the water wall is 20° C., the temperature difference is 10° C., and the rate of heat loss is 116 W. Each hour, the heat loss is 417,000 Joule, or about 500 BTU.
On the other hand, suppose the size of the water wall is 5 ft high 7 ft wide and 6 inch thick. The volume of water is about 0.5 cubic meters. The heat capacity of the water is 4.19×500,000=2,100,000 Joule/° C. Each hour, the temperature drop is 0.2° C. For 10 hours, the temperature drop is 2° C. Therefore, one half metric ton (or approximately one half English ton) of water is enough to keep the temperature of an average-sized, well-insulated room comfortably constant. It is important to note that a 0.5 metric ton water wall, comparable with the weight of a grand piano, is not ax extraordinary burden for an average house. Especially, the water wall is located near the wall, not near the center.
Next, we estimate how much solar power is required to cool the water wall down by at least 2° C. during the day time. We assume that the external temperature is 10° C. higher than the temperature of the water wall. Using a refrigerating unit with a coefficient of performance (COP) of 6 (see for example W. P. Jones 1994), to bring the temperature of the water wall with heat capacity of 2,100,000 Joule/° C. down by 1° C., an input energy of 2,100,000×10/6=3,500,000 Joule is required to operate the compressor. If the process takes 8 hours, the average power required is 3,500,000/(8×3600)=121 W. To cool down the water wall by 2° C., 242 W of average power is required. Therefore, a 500 W peak power is sufficient. The size of a typical 200 W solar photovoltaic panel is 1 m×2 m. Two 200 W solar photovoltaic panels is of a perfect size to function as an awning for a pair of 3-ft times 4.5-ft windows.
Finally, we estimate the solar power required to heat up the water wall during the winter. The solar constant is roughly 1 kW per m2. The area of two standard windows (3 ft times 4.5 ft) is 2.5 m2. Suppose that the transmissibility of atmosphere and the glass is 60% and the absorbance of the (black) water wall is 80%, the solar power received by the structured water wall is about 1 kW. If the average solar exposure is 6 hours with a sinusoidal profile, the heat absorbed is 10,800,000 Joule, which can heat up the water wall by about 5° C. However, at the same time, there is heat loss though the walls and the windows. The net rise of the temperature of the water wall is about 2-3° C. During the night, the temperature of the structured water wall gradually decreases by about 2-3° C.
The maximum cooling power and heating power by design is preferred to be greater than for maintaining a comfortable temperature. To avoid excess cooling or excess heating, the operation is regulated by a thermostat. In case of cooling, when the temperature of the structured water wall is lower than a predetermined value, for example, 17° C. or 63° F., the refrigeration unit is turned off. In case of heating, when the temperature of the structured water wall is higher than a predetermined value, for example, 23° C. or 75° F., the insulating shutters are closed. In a cool and cloudy winter day, if the temperature of the structured water wall is too low, an external heater can be tuned on.
