Patent Application
20090301118
Cooling of buildings consumes a huge amount of energy worldwide. For example, in southeast and southwest regions of the US, air conditioning is the dominant end user of energy and the single leading cause of peak demand for electricity. From economics point of view, reducing electricity demand for space cooling could save a lot for consumers. From utility-infrastructure point of view, reducing air-conditioning electricity loads can lower demand for annual power generation and peak capacity. Coincidentally, in regions where air conditioning is needed the most, solar energy is also abundant. Especially, from day to day, the stronger the sunlight, the more air conditioning power is needed. Therefore, to utilize sunlight to power air conditioning is a logical solution.
A basic problem of solar-powered air conditioning is energy storage. Solar energy is only available in sunny days, not in evenings and nights. By directly using the solar electricity to drive an air-conditioning unit to cool the rooms, 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 solar power 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). In the evenings and nights, although air conditioning is still needed, there is no sunlight. Sunlight is also absent in cloudy and raining hours. To resolve this problem, an established method is to use solar energy to generate hot water, and then to drive an adsorption refrigerator using the stored hot water. Using a good thermal-insulated tank, hot water can be stored for a day or two. However, the efficiency of the adsorption refrigerator is low, and the stored hot water would radiate heat into the building and to increase the demand for cooling. Thus, the approach has not been widely implemented.
A method to store cooling power using ice has been proposed in the 1980s, with the purpose of taking advantage of the price difference of electricity in peak time and night, mostly for large commercial buildings. The problem with that approach is that when water is frozen to ice, the volume is increased, and the expanding ice could damage the container and the heat-exchange coils. That method has not been widely implemented also.
The recent advancements of renewable energy technology have opened new possibilities for a practical solar-powered air conditioning system. The first is the rapid reduction of the price of solar cells, which is approaching one dollar per watt, and the price will continue to drop. Consequently, the cost of generating DC electricity using solar cells per watt is only a fraction of the cost for construction, maintenance and management of electricity transmitting facilities. To drive appliances directly from solar electricity is economically advantageous. The second one is the availability of inexpensive crude glycerin as a byproduct of biodiesel. To produce each gallon of biodiesel, 0.15 gallon of surplus crude glycerin is generated. Although high-purity glycerin has applications, purification from crude glycerin is expensive. The limited market for pure glycerin does not always justify the cost to construct glycerin purification plants, which leaves about one million metric tons of crude glycerin for disposal annually.
The present invention is related to a design of a solar-powered air conditioning system which utilizes the DC current directly from the solar cells to drive a vapor-compression refrigeration unit to freeze a mixture of glycerin and water (typically 5%-10% of glycerol and 90%-95% of water), then use the stored cooling power of the partially frozen glycerin-water mixture to cool the building. Because the low cost of crude glycerin, the elimination of the inverter and power distribution facilities, and the simplicity of the entire apparatus, the solar-powered air conditioning system is economically advantageous.
The current invention is a solar-powered air conditioning apparatus with a safe and inexpensive medium to store cooling energy. It utilizes the DC current from solar cells to drive a vapor-compression refrigeration unit directly, bypassing the inverter and power grid. It utilizes a mixture of glycerin and water to store cooling energy. The cooling coil of the refrigeration unit is placed in a container filled with a mixture of glycerin and water, thus to freeze it into slurry of small pieces of ice embedded in an aqueous solution of glycerin. Experiments showed that with a mixture of 5% glycerin and 95% water, the frozen substance resembles wet snow. With a mixture of 10% glycerin and 90% water, the frozen substance is a fluid filled with small ice sheets. With a mixture of 7.5% glycerin and 92.5% water, the frozen substance is a viscous fluid packed with ice sheets and pellets. Because it is fluid, it would not damage the container and the heat exchange coils. On the other hand, the latent heat of the ice sheets and pellets is as large as regular ice, which is 80 calorie per gram, or 335 kilojoules per kilogram. The temperature is a few degrees below the freezing point of water. A stream of air is passing through a set of heat exchange tubes in the frozen mixture of glycerin and water, and then circulates by a fan or a blower to cool the space. Using a thermostat, the temperature can be regulated through the fan or the blower, similar to ordinary air conditioning units.
As an apparatus in a residential home or an office, safety is a major consideration. Glycerin is a popular food ingredient and a commonly used skin-care lotion, therefore it is very safe.
Cost is an important factor for the viability of a product. Decades ago, glycerin was fairly expensive. In recent years, a glut of crude glycerin exists as a byproduct of biodiesel. The market price of crude glycerin is $0 to $150 per metric ton. The impurities in crude glycerin are mainly rock salt, water, alcohol and residual fat, which are not toxic. Therefore, crude glycerin can be directly used without purification.
For practical reasons, here we make an estimate of how much energy-storage medium is required. For the case of a relatively large single room, using standard insulation, if the outside temperature is 30° C., to maintain a room temperature of 20° C., the rate of heat loss is 200 W. Each hour, the energy loss is 720 kJ. If the mass of the thermal-storage medium is 100 kg, assuming one half of the mass is frozen to ice, the latent heat is 1.67×104 kJ, and the thermal-storage medium can maintain the temperature for 24 hours. For an entire house, for example, equivalent to five relatively large rooms, a 500 kg thermal-storage medium could maintain the temperature for 24 hours.
For financial reasons, the size of solar panels is estimated as follows. Suppose the coefficient of performance (COP) of the refrigerator is 3-5, to freeze one half of the 100 kg glycerin-water mixture in 5 hours, a power of 180 W to 300 W is required. If the price of solar cells is one dollar per watt, the cost of solar cells is $180 to $300. Assuming that the efficiency of solar panel is 10%, the area of solar cells is 1.8 square meters to 3 square meters. For a central air conditioning system, the cost of solar cells is five times higher: 0.9 kW to 1.5 kW with a cost of $900 to $1500 and an area of 9 to 15 square meters. Comparing with the savings of electricity, the cost and the roof area is reasonable.
An important issue is transportation and installation of the thermal-storage medium. Because water is available anywhere, the factory could supply an empty container and crude glycerin (for example 80%). For a single room, the required quantity of crude glycerin is less than 10 kg, which can be shipped in two one-gallon plastic bottles. At the site, crude glycerin and water is mixed in the container. For an entire house, the required quantity of crude glycerin is less than 50 kg. Since on the open market, the price of each ton of crude glycerin is less than $150, the cost of crude glycerin for a single room is less than $1.50 and cost of crude glycerin for a central air conditioning system is less than $7.50.
