IdeaSagar: Solar Coolhouse Concept

Solar Cool Houses in the Desert

{A patent spec prepared and then postponed for the time being.}

Abstract

Solar Cool-houses are like traditional greenhouses but a significant difference is that they create a cool environment for plants to thrive in an otherwise harsh (hot and dry) environment. They will have transparent roofs covered with transparent solar photo voltaic (PV) panels and hot water heaters. During the day under the sun, the transparent PV panels produce electric power which can be used to generate chilled water or cool air to keep the greenhouse environment cool. Locally available non-potable water or sea water is heated in the water heaters and through desalination process, convert it to water suitable for plants.

As global population increases, there is a greater demand for food, water, and energy. Solar photovoltaic (PV) systems are increasingly used as energy generators. But, such systems occupy large tracts of land competing for land, which is needed to grow more food. Solar cool houses can be built on the vast tracts of currently waste lands that are desert like, fallow, and barren. Thus, such lands in the harsh, hot, and dry desert-like environments can be converted into flourishing crop lands.

By a proper selection of materials for the solar cells for the PV panels, the wavelengths of light in solar spectrum that are utilized for photosynthesis are passed through to the plants in the cool-house. Thus, one hundred per cent of the energy in the solar radiation incident on the roof is absorbed by the PV panels, the plants, and the water in the hot water heaters.

Any excess or deficit of electric energy generation in a solar cool house complex is exchanged with the electric power grid. Deserts and arid lands usually have very inferior soils. In such locations and situations, hydroponics, and pisciculture can thrive in the cool houses.

BACKGROUND

Field of the Invention:

This invention relates to solar coolhouses to create a suitable environment to grow plants in an otherwise harsh hot and dry desert-like environment while generating electric power with transparent solar photovoltaic (PV) panels on top of the coolhouse roofs.

The present invention describes a coolhouse that has transparent solar PV panels fixed to a structure with a transparent roof to create an environment suitable to grow plants in the coolhouse. Solar radiation of certain wavelengths is essential for photosynthesis, a biochemical process through which plants take advantage of sunlight and generate life sustaining vegetable foods. But the severity of the solar radiation in a desert is highly detrimental to vegetation. Another aspect of the present invention is directed towards using a part of the energy from the solar spectrum to generate electricity through transparent photo voltaic (PV) panels on the roofs, using another part of the solar radiation to heat sea water for desalination, and allowing the rest of the solar radiation spectrum to be used by plants for photosynthesis. This effect is accomplished by properly shielding the crops from the full impact of direct solar radiation and by distributing the solar radiation by suitably building coolhouses to allow plants to thrive. The electric energy generated by the transparent photovoltaic panels is used to generate clean water needed by the plants from sea water or hard water by desalination and to run the air conditioning equipment to create a thriving environment in the coolhouse and any excess or deficit of electric energy generation is exchanged with the electric power grid.

Deserts and arid lands usually have very inferior soils. In such locations and situations, it is proposed to use the coolhouses for hydroponics, and Pisciculture.

Description of the Related Art:

As global population increases, humans depend on every acre of land and every gallon of fresh water to raise food needed by the billions of people in the world. Life sustaining food results from agriculture, which critically depends on land, fresh water, energy, solar radiation, and proper environment.

As they run out of fertile farm lands, nations have been trying to cultivate new lands that are dry, arid, fallow, and desert like harsh environments. Constant efforts are being made to bring new sources of water including desalination of seawater and reuse of dirty and hard water. New and affordable sources of energy, where available, are being applied to improve agriculture. Technology and designs to augment solar radiation in extremely cold as well as hot and dry climates are being applied in concert with other methods to improve agriculture and gain better yields.

In cold climates, greenhouses are built to augment the local environment to extend the periods when vegetables and fruits can be grown. Greenhouses are also known as glasshouses, hothouses, or coldframes. These structures are covered with a transparent layer made of plastic film, rigid plastic, or glass. The cover permits natural light to enter to allow plant growth but prevents cold air to enter or warm inside air to exit. The inside temperature is maintained by heating with an external source as needed. Although most of these efforts are expensive in terms of initial costs, cultivation of proper vegetables, fruits, flowers, and other crops results in attractive economic returns.

Desert farming generally relies on irrigation with the Imperial Valley in California is a good example of successful desert farming. One problem associated with raising traditional plants in a desert is depletion of the ground water. Drip irrigation is one way to reduce the overall water demand especially in desert areas.

The geography of Israel is not naturally conducive to agriculture as more than half of the land area is a desert, the climate is harsh, and the country lacks water resources. With hot daytimes and cold night times, hothouse technology is used to grow vegetables in that country.

SolarSpring developed clean-energy water systems in remote areas by using solar photovoltaic systems to desalinate and treat seawater and non-potable water. Solar thermal energy is used to heat and evaporate seawater or dirty water and the water vapor is subsequently cooled to produce clean water by a process called, solar humidification-dehumidification (HDH) process. The energy required for pumping the water is powered by solar photovoltaic panels. The creation of fresh water by the desalination of water using the waste heat from Concentrating Solar Power (CSP) plants—a welcome bonus in arid regions.

In the interior areas of continents, other water sources, such as, groundwater with mineral contaminants (e.g. Arsenic), degraded wells, and streams with mineral contamination, are available but cannot be used directly for agriculture. Zonnewater BV developed an optimized solar thermal distillation system. It is intended to produce premium drinking water as well as water for agricultural applications from different available sources in tropical and sub-tropical locations.

A pilot plant built by the Sahara Forest Project (SFP) produced 75 kilograms of vegetables per square meter in three crops annually, comparable to commercial farms in Europe, while consuming only sunlight and seawater. SFP designed a special greenhouse in Qatar. At one end of the greenhouse, salt water is trickled over a grid-like curtain so that the prevailing wind blows the resulting cool, moist air over the plants inside. This cooling effect allowed the facility to grow three crops per year, even in the scorching summer. At the other end of the greenhouse is a network of pipes with cold seawater running through them. Some of the moisture in the air condenses on the pipes and is collected, providing a source of fresh water.

While traditional methods of agriculture use vast quantities of water, and fertile soil, hydroponics utilizes several times lower quantities of water and uses no soil at all. Plants are grown in inert medium or in water itself. Of course, in either case, liquid nutrients are added to water.

Pisciculture improves on hydroponics as it allows both fish and crops to grow together. The plants thrive by using the rejects from fish as supplementary nutrients and the fish use parts of the plant as food. Pisciculture also uses less water than traditional agriculture.

With issues of climate change and greenhouse gases putting pressure on fossil fuel-based thermal plants, solar photovoltaic energy generation is gaining favor. But, this trend would take away vast areas of land from agriculture when the PV plants are situated on ground. There is a hope that there is potential for growing plants for food and other uses in the shaded areas under the solar mirrors. It is also possible to use desalination of sea water to water the plants. But without full enclosures, hot winds and fierce sand storms will still cause problems for plant cultivation in the deserts.

Every year, each square kilometre of desert receives solar energy equivalent to 1.5 million barrels of oil. Multiplying by the area of deserts worldwide, this is several hundred times as much energy as the world uses in a year. Hence, there is a constant effort to utilize solar energy fully and benefit from it.

Figure 1 shows the solar radiation spectrum. Solar radiation spans wavelengths from 250 nanometers to 2,500 nanometers covering the Ultra Violet (UV), Visible, and Infra-Red (IR) regions. Plants use only some of the light in the 400–700 nm range. Hence, 47% of the incident solar radiation comprises non-bioavailable photons, which is not useful for photosynthesis of the plants but is converted to heat under normal circumstances.

Of the global radiation reaching Earth’s surface as shown in Figure 1, on average, 45% of the incoming solar radiation falls within the range of 389 to 710 nm. This is the range utilized via photosynthesis by plants. This range is often defined as photosynthetically active radiation, PAR, and is often denoted by the range between 400 to 700 nm as shown in Figure 2. It may further be noted that of the 100% total energy received by the leaf, only 5% is converted into carbohydrates and later for biomass production. The amount of energy lost by nonabsorbed wavelengths is thus 60%; this energy is reflected by the plants into the surrounding atmosphere.

Figure 2 also shows the response of various types of chlorophyll in plants to light. This examination of the characteristics of plants reveals that plants use sunlight within the visible range and even within this range, there is a certain capacity that plants can use and the rest of the light incident upon them is dissipated as heat. Plants differ in the types of chlorophyll they use in using sunlight, a fact worth noting in the selection and design of agricultural or crop systems.

When we examine the solar to electric energy conversion mechanisms, it is known that when light is incident on a material, it is partially reflected, absorbed, and transmitted. Only that portion, which is absorbed, is converted to electricity in the photovoltaic panels. These fractions depend on the materials. There is constant research and development to discover and formulate new materials with high absorption values. These absorption values are typically dependent on the wavelength of the light impressed upon them. Thus, it is possible to select combinations of electronic materials that absorb solar radiation at wide wavelengths but allow light of certain selected wavelength range to pass through for other potential uses.

Figure 3 shows the spectral response of a selected set of popular materials used for solar cells. Gallium Arsenide (GaAs), Cadmium Telluride (CdTe), amorphous Silicon (a-Si), crystalline Silicon (c-Si), and Copper Indium Selenide (CIS) are some of the materials used in solar panels and it may be noted that each material absorbs light (photons) within certain specific wavelengths of the radiation and converts it into electrical energy. The rest of the radiation is passed through the solar panel in transparent materials. With most materials currently in use, the amount of energy absorbed for conversion to electricity is only a small fraction of the incoming radiation. With solar panels operating with efficiencies in the range from 6% to 18%, it means that a large fraction of incident photonic radiation is available past the solar panels. For example, (CIGS) cells on glass give efficiency between 18.1% and 14%, and flexible CIGS cells on polymer and metal foils give 16% efficiency, solar cells with non-vacuum low cost CIGS process give 6.7% efficiency, CdTe cells on glass give 15.4% efficiency, and flexible CdTe cells on polymer give 12.4% efficiency. In almost all these cases of solar panels, a high proportion of the incident radiation is available for other uses.

Indium tin oxide (ITO) can be used in nanotechnology to provide a path to a new generation of solar cells. Solar cells made with these devices have the potential to provide low-cost, ultra-lightweight, and flexible cells with a wide range of applications. Because of the nanoscale dimensions of the nanorods, quantum-size effects influence their optical properties. By tailoring the size of the rods, they can be made to absorb light within a specific narrow band of colors. By stacking several cells with different sized rods, a broad range of wavelengths across the solar spectrum can be collected and converted to energy. Moreover, the nanoscale volume of the rods leads to a significant reduction in the amount of semiconductor material needed compared to a conventional cell. Thus, ITO solar cells can generate electricity and also allow light at frequencies useful for photosynthesis by plants.

Thin film photovoltaic cells are made by depositing one or more thin layers of photovoltaic material on a substrate and they offer lower production costs and acceptable efficiency values. The sub-groups of thin film photovoltaic cell include amorphous Silicon (a-Si), Cadmium Telluride (Cd-Te) and Copper Indium Gallium Selenide (CIS or CIGS).

Polymer solar cell (PSC) produces energy by absorbing infrared light and is not visible. The cells are thus nearly 70 percent transparent to the human eye. Made of photoactive plastic, the cells have been seen as highly viable solutions for building integrated solar installations. They are fabricated in long sheets suitable for building materials. Such Building Integrated PhotoVoltaics (BIPV) will suit special applications in greenhouses.

By taking conventional opaque PV materials and either making them so thin they are translucent or “segmenting” them, “see-through” solar cells could be made with some tradeoff between transparency and efficiency. Such a tradeoff is not entirely negative if the transmitted energy in the light is used by the plants and the hot water heaters. From Figure 4, we can see that transparent solar cells fulfill these requirements admirably. These cells utilize the UV and near-infrared (NIR) light to generate electric energy and pass the visible light through.

MIT researchers are making transparent solar cells (Figure 4) that could turn everyday products such as windows and electronic devices into power generators—without altering how they look or function today. These solar cells absorb only infrared and ultraviolet light. Visible light passes through the cells unimpeded. Using simple room-temperature methods, the researchers have deposited coatings of their solar cells on various materials. They estimate that using coated windows in a skyscraper could provide more than a quarter of the building’s energy needs without changing its look.

Michael Grätzel produced the most efficient Perovskite solar cells, which convert 15 percent of the energy in sunlight into electricity. It is expected that future cells made with this type of materials could result in efficiencies as high as 20 to 25 percent. Perovskite solar cells can be made by spreading the pigment on a sheet of glass or metal foil, along with a few other layers of material that facilitate the movement of electrons through the cell. With such photovoltaic materials it is possible to paint or spray solar cells on transparent roofs. Transparent PV panels are available commercially as fabric and a commercial sample is shown in Figure 5. It is also possible to stack up such fabric based solar cells over water heaters made with the transparent fabrics.

The performance of PV cells improves when they are cooled. Hence, by locating a hot water heater below the transparent solar PV panels, we accomplish three things: 1) the performance of the PV panels is improved; 2) the intensity of harmful solar radiation reaching the plants is reduced; and 3) the heat recovered by the water is usefully utilized in the desalination process.

The integral passive solar water heaters (IPSWH) are typically rectangular plastic bags holding water for heating. These were popularly called "solar pillows" A similar model, with a simple tray instead of bags, has also been used. The point is to minimize the thermal energy reaching the plants in the desert environment and where possible to utilize the collected thermal energy.

It is known from the above description that 1) solar photovoltaic systems are not 100% efficient; 2) they do not use the solar radiation present in the entire spectrum of sunlight; and 3) plants do not use all the sunlight they receive. Hence technologists look for ways to develop systems that take advantage of the mutual inefficiencies and indifference of system components to parts of the light spectrum.

Bhatt (US Patent 5,101,593) described a Portable greenhouse working on solar system. He disclosed a closed chamber with windows that permit the entry of light into a major part of the chamber where plant growth will occur while restricting light to a minor portion thereof where germination of seeds occurs. Racks for stacked growing trays are provided; preferably the upper part of the chamber is used for germination. The greenhouse further includes apparatus for accumulating solar energy in the form of electrical energy so as to drive irrigation pumps and fans and also in the form of thermal energy to assist in maintaining desired temperatures in the chamber.

Hajj airport terminal uses for its roof a Birdair provided translucent membrane material to shield the interior from the high temperatures in the desert. That material keeps 70% of the light from entering the interior.

SUMMARY

One object of the invention is to develop an enclosure to allow only a controlled portion of sunlight to impinge on the plants inside the said enclosure by providing a transparent roof covered by successive layers of transparent solar photovoltaic panels and transparent solar hot water heaters to produce fresh water from sea water by desalination, which fresh water is used to feed the plants raised in the said enclosure, and to create controlled temperature and humidity conditions suitable to raise vegetation by supplying cooled air produced by an air conditioning or chiller system powered by using the electricity generated by the above mentioned solar panels and augmented by grid power.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows the solar radiation spectrum

Figure 2 shows the rate of photosynthesis and the amounts of sunlight in the various wavelengths that are absorbed by the chlorophyll in plants.

Figure 3 shows Spectral Response of solar cells.

Figure 4 shows Spectral response of conventional and transparent PV cells

Figure 5 shows a sample of a commercial Thin Film solar PV in fabric form

Figure 6 shows how stacking takes advantage of the solar radiation spectrum and the spectral response of photovoltaic panels.

Figure 7 shows how the wavelength spectra can be matched so that the solar radiation can be appropriately shared and utilized by the series of devices.

Figure 8 shows a view of stacked solar PV panels and solar hot water heaters on a transparent roof of a solar coolhouse.

Figure 9 shows a sketch of the general arrangement of the solar coolhouse.

Figure 10 shows a schematic arrangement of the solar coolhouse.

Figure 11 shows a view of the ventilation arrangement in the gable portion of the roof of the coolhouse.

Figure 12 shows a coolhouse with the addition of a rock bed recuperator.

Figure 13 shows a solar coolhouse with hydroponics or aquaponics instead of soil based plants grown inside the solar coolhouse.

Figure 14 shows a solar coolhouse with pisciculture cultivation.

DETAILED DESCRIPTION

Figure 6 explains the principle of the arrangement where transparent photovoltaic (PV) panels (12), transparent water heaters (18), and transparent roof (26) are placed in series. With such an arrangement, solar radiation (10) passes through the transparent PV panels (12) generating electric energy (14) and the unused solar radiation enters the transparent water heater (18) where cold water (20) is heated and the heated water flows (22) to a desalination system, and the remaining solar radiation (24) finally enters the solar coolhouse through the transparent roofing (26) to reach the plants (30) inside the coolhouse (28).

By suitably designing and arranging the components of the system as illustrated in Figure 7, we take advantage of the fact that transparent PV panels and photosynthesis by plants utilize solar radiation at different wavelengths, while the water passing through the water cools the PV panels by absorbing most of the remaining thermal radiation. This apportionment of solar energy in various wavelengths is further explained in Figure 5.

It is possible to take advantage of the spectral responses and absorption and transmission portions of light at various wavelengths, in other words, mutual inefficiencies and indifferences, by locating the PV panels, water heaters, and plants successively. As shown in Figure 7, by placing transparent PV cells (50) with characteristics that allow some of the solar radiation in the visible wavelengths shown at (40), (46), and (42) to pass through and the solar radiation in the UV (44), part of the visible wavelength portion of light in a particular region (46), in the Near InfraRed (NIR), and InfraRed (IR) wavelengths (48) to be absorbed for conversion to electricity, we get the most beneficial effect of the solar radiation. Since the transparent solar PV cells absorb only a small portion of the solar radiation incident on them, the rest is passed through. A greater part of such pass through solar (thermal) radiation is absorbed in the hot water heater constructed of transparent material such as glass or plastic (52) and the reminder passes through the transparent roof (54) to the plants in the coolhouse. Thus the plants can get a large portion of the light (56) and (58) in the two visible regions. Small amounts of radiation in the UV region (60), NIR (62), and IR wavelengths (64) might also reach the plants depending on the absorption capabilities (or efficiencies) of the PV cells and hot water heaters. The choice of materials for solar PV cells in the panels will be optimized to minimize the radiation reaching the plants in the UV (60), NIR and IR (64), and the unusable (by plants) visible region (62). As new transparent solar PV materials and cells are discovered new combinations can be matched as closely as feasible, practicable, and economically viable with the frequencies that are preferred by plants in order to increase the utilization of natural solar radiation to generate electricity and to encourage growth of plants and to minimize the need for hot water heating.

Following the above discussed principles, we design a stacking arrangement, as shown in Figure 8, where transparent solar PV panels (160) and solar hot water heater (162), are stacked on top of the transparent roofing (164) of the coolhouse.

In order to utilize most of the solar radiation effectively, we design a solar coolhouse or a greenfarm in a desert as shown in Figure 9. The arrangement (170) consists of a structure with opaque walls (172) on all four sides and a transparent roof (164) on top. Solar Photovoltaic panels (160) and solar hot water heaters (162) are stacked on top of the said roof (164). Electricity (174) is generated using the transparent solar photovoltaic panels (160), some of the electric power generated is utilized for local use (176) and the rest of the energy is exported to the electric grid (178). Seawater or local brackish water (180) is sourced and pumped (182) into the solar hot water heaters (162). The hot water is conveyed to a desalination unit (184), where freshwater (186) and concentrated brine (188) are separated. The resulting concentrated brine from the desalination plant is transported to a salt production facility or suitably disposed of outside the coolhouse. An air conditioning unit (190) powered by a part of the electricity generated by the solar panels supplies cool fresh air (192) to keep the environment in the coolhouse at the required temperature and to replenish the carbon dioxide (CO₂) used by the plants. The floor (200) of the structure is prepared with soil (202) and serves as a farm or bed for plants. Suitable plants and crops (204) are raised on the farm or soil bed. Fresh water (186) generated by the desalination unit is sprinkled (206) on the soil and sprayed (208) into the structure to provide controlled moisture in the soil and humidity in the coolhouse environment suitable for plants and crops.

Figure 10 shows a schematic arrangement of the Solar Coolhouse. In this arrangement (210), solar radiation (212) is incident on the transparent solar PV panels (214), a part of which passes through the panels on to solar water heaters (216) where a portion is absorbed, and the remaining radiation passes through the transparent roof (218) and into the cool house (220). The electric power generated by the transparent solar PV panels is taken to a junction box (222) where a portion of the power is exported to the electric grid (224). When solar radiation is not available, the PV panels do not generate power and power is actually imported from the electric grid (224). Sea water or other brackish water that is locally available is stored in a reservoir (226) from where that water is pumped (228) to the solar hot water heaters on the roof (216). The heated sea water or brackish water is sent to the desalination system (230). Concentrated brine from the desalination system is collected in drains (232) and returned to salt recovery plants where possible or otherwise disposed of. The desalination system (230) gets its electric power requirements from the electric junction (234), which is connected to the junction box (222). Plants (236) are grown inside the coolhouse. The air conditioning and ventilating system (238) supplies cool air to the coolhouse and vents the hot air and thus ensures that the environment inside the coolhouse is maintained at the required temperature and humidity to ensure good plant growth. A water pump (240) pumps fresh water generated by the desalination system (230) into the coolhouse (220) to water the plants (236) by means of sprinklers and sprayers.

The fresh water produced by the desalination system is used to water the plants by means of sprinklers or drip pipes and additionally, some of that water is sprayed into the air in the coolhouse to keep the humidity at levels required for good growth of the plants.

Where an electric grid connection is not available, a battery back-up system is used to store electrical energy during the sunny days for use in the nights and on days when the sun is not shining.

Carbon dioxide (CO₂) can be rapidly depleted at the leaf surface as plants depend on carbon dioxide for their growth. A slight wind is necessary to replenish carbon dioxide (CO₂) near the plant surface (236). In a fully enclosed coolhouse, fans will be used to bring outside air to replenish carbon dioxide (CO₂). Hence, by providing ventilating blowers in the gable portion of the coolhouse, the twin purposes of replenishing carbon dioxide (CO₂) and venting the hot air are accomplished.

Figure 11 presents a view to show the ventilation arrangement. The solar radiation (240) incident on the structure consisting of transparent solar PV panels and hot water heater panels covering the transparent roof (242) will be likely to cause the air in the solar coolhouse (244) to heat. Some external heat will penetrate the structure (246) through the parts of the roof (248) where no solar panels might be located and the walls, and infiltration of outside air through the entry doorways. Such hot air will accumulate in the gable portion and increases the load on the air conditioning system if it is not vented. Ventilating blowers or fans (250) are optionally installed in the gable portion of the structure. By turning these fans on, air that is hot and deficient in carbon dioxide (CO₂) can be ventilated and the temperature inside the structure can be kept at the desired level without placing undue burden on the air conditioning system.

In certain deserts, while the day time presents scorching temperatures, the night time temperatures dip too far and it becomes very cold for the plants. Figure 12 shows an optional chamber (260) with a bed of rocks (262) added to the coolhouse. This bed of rocks works as a recuperator. During the day, a fan (264) draws the prevailing hot air from the outside through the bed of rocks. The resulting heat exchange heats the rock bed and cools the incoming air. Such cooled air is admitted into the coolhouse. During the night time, when cold air from the outside air is pulled through the hot rock bed, the air warms up before it enters the coolhouse. The rock bed gives up its heat and cools itself. The air entering the coolhouse in the night time presents no harm to the plants in the coolhouse as it is warmer than the outside. Such a recuperative cold store will further save energy. With such controlled recuperative arrangements, the environment in the coolhouse is maintained within limits that allow plants to thrive while lowering energy expenditures on air conditioning. In the event the desert nights present severe cold temperatures, the air conditioning system can be operated in reverse as a heat pump to maintain the required conditions in the coolhouse to protect the plants from cold freeze.

In the solar coolhouse arrangement described so far, transparent solar PV panels and solar hot water panels are placed on the side where solar insolation is predominant. The numbers, sizes, capacities, and placement coverage of the solar PV panels and solar hot water panels are determined at the time of design by the actual requirements of hot water, solar PV electric energy, and the amount of direct sunlight required by the plants inside the Coolhouse.

In the Solar Coolhouse arrangement described above, the floor is a bed of soil where plants are grown. In deserts the quality of soil is poor and to compensate for that we go for hydroponic cultivation. An alternative arrangement consists of providing hydroponics to grow crops in the Solar Coolhouse as shown in Figure 13. In the solar coolhouse (170), instead of soil on the floor (200), water containers (270) are located with plants (272) growing in them. Fresh water from the storage reservoir (184) is drawn via pipes (186) by the water pump (274) and pumped via pipes to the water containers (276). Water leaves the containers via outlet pipes (278) to return to the fresh water storage unit via pipes (280) and to the water sprayers (208) in the cool house to ensure proper humidity in the solar coolhouse. Appropriate mineral nutrient solutions are added to the water flowing in the containers. Appropriate mineral nutrient solutions and fish food are added to the water flowing in the containers. While a variety of vegetables, flowers, herbs, and plants can be grown in solar coolhouses equipped with hydroponics, traditional facilities have successfully grown Artichokes, Asparagus, Beans, Beets, Broccoli, Brussel Sprouts, Cabbages, Carrots, Cauliflowers, Celery, Cucumber, Eggplants, Leeks, Lettuce, Onions, Parsnips, Peas, Potatoes, Radishes, Rhubarb, Squash, Tomatoes, and Yams.

Yet another alternative arrangement consists of providing Aquaculture or Pisciculture where fish are raised in the water-filled troughs with plants being grown in the same water-filled troughs as shown in Figure 14. Water-filled containers (290) are used to grow plants (292) and also to raise fish (294). Fresh water from the storage reservoir (184) is drawn via pipes (186) by the water pump (274) and pumped via pipes (276) to the water containers (290). Water leaves the containers via outlet pipes (278) to return to the fresh water storage unit via pipes (280) and to the water sprayers (208) in the cool house to ensure proper humidity in the solar coolhouse. Appropriate feed materials are added to the water in the tanks for the fish to flourish and for the vegetable plants to thrive. Besides vegetables, a variety of fish species including salmon, catfish, tilapia, cod Katla, Rohu, Mrigal, and exotic or common carps can be raised in aquaculture or pisciculture in a solar coolhouse.

Several alternative arrangements and configurations are possible within the scope of the description and some typical alternatives are described below.

In some deserts, the night time temperatures will be very low. Hence, water storage units and heat exchangers can be used to improve the performance of the solar coolhouse system. By passing seawater through the roof top heaters in the night time, water can be cooled to low temperatures and saved in water storage tanks. During the daytime, the stored cold water can be passed through the roof top heaters to heat the seawater. Auxiliary water to air heat exchangers can be used to use the stored cold water to cool the air being drawn into the coolhouse. Such a subsystem will reduce the load on the air conditioner and during certain times of the day and season, it may eliminate the use of the air conditioners completely. Likewise, the same water to air heat exchangers can provide temperate air into the solar coolhouse. This heat storage in water will save energy used in providing the proper environment in the coolhouse.

With the electrical energy generated or imported from the electric grid, it is possible to extend the hours when the plants get light by adding electric grow lamps. Such an arrangement will allow for rapid and robust growth and crop yields.

By incorporating the solar humidification-dehumidification (HDH) process additionally, seawater or local brackish water can be sprayed as a mist and the resulting fresh water can be collected and used to water the plants in the coolhouse.

Seasonal and daily variations in temperature and solar radiation would vary the need for electrical energy by the coolhouse environment conditioner. Connection to electrical grid would help the system and economics. Where such a connection to the electrical grid is not available or economical battery storage will help.

Desalination of seawater or locally available hard water can be done by raising steam in auxiliary concentrated solar heaters.

In cold deserts, such as Ladakh region in India, where water and good soil for cultivation are scarce, solar heating can be used to melt the ice and snow recovered in the winter and hydronic and pisciculture can be utilized with solar coolhouses to raise plants and fish. Such ice can also be used to cold pack fruits and vegetables and export them to other regions.

A computer operated environmental control subsystem will assist in the proper operation of the solar coolhouse.

Following developments in solar photovoltaics on clothing, transparent photovoltaic thin films can be glued to a semi-transparent roof of the coolhouse structure in the desert.

The side of the solar coolhouse that has no exposure to the sun will have a roof made of opaque material and will carry no solar photovoltaic panels. It may be equipped with wind turbine ventilators to aid in controlled ventilation of the hot air from the gable portion of the roof of the coolhouse.

Where seawater is not available but fresh water is available, desalination process will be eliminated and such fresh water in place of seawater in the water heaters on the roof of a coolhouse.

It is also possible to utilize designs which do not use water heaters at all while sacrificing some advantages.

It is possible that by controlling the rate of amount of flow of seawater or other medium in the heaters on the roof of the coolhouse during bright sunny to cloudy days to augment the amount of light energy reaching the plants to optimize their growth and development.

It is possible to use an intermediate clear fluid such as polyethylene glycol in the solar heaters on the roof and to transfer that heat from that fluid to the seawater prior to its entry to the desalination system. This arrangement will have the advantage of using clean intermediate fluid rather than the seawater which may be dirty and turbid.

Since the plants in the structure do not depend on the external atmosphere, plants can be grown throughout the year. The economics can be highly favorable as the roof structure is doubling as the energy generator and as a shield from the hot sun in a desert for the crops inside.

Where necessary skylights can be added to the roof to allow additional sunlight to enter and shine on the vegetation in the coolhouse. In such a case, it is possible that only certain spots receive sunlight and not all the plants receive light uniformly. In order to rectify such situations, Fresnel lenses may be added to the skylights so that the sunlight is distributed to all the plants in the structure.

Where solar thermal radiation is severe and the plants do not need the sunshine for certain periods of time, black fabric shields or curtains can be pulled on the inside of the coolhouse roof to reflect the sunlight away from inside thus saving on the energy required for air conditioning. Likewise in certain deserts where poor cloud cover exposes the environment to the blue night sky and cause very low temperatures to prevail. In such situations, black fabric shields or curtains can be pulled inside to protect the plants from freezing inside the solar coolhouse.

It may be noted that the space in the coolhouse can be used as a warehouses for grains and other food products, instead of or in addition to raising crops. With such uses, solar coolhouses can find home in locations with incessant or intermittent rainfall.

Although we described coolhouses here, their sizes can be extended to large values to build solar cool farms or greenfarms on the one hand and the concept can be applied to cool boxes on the other so that people can grow plants in their kitchen windows.

We described the concept as applicable to deserts but it does not preclude the use of solar coolhouses on the seas. Especially pisciculture may be advantageous in offshore locations.

Solar coolhouses can be located on top of multistory homes for penthouse gardens as they give a cool outdoor garden atmosphere for plant growth as well as to yield vegetables, fruits, and flowers in otherwise harsh and unwelcoming environments.

Just as people have gardens around their houses, they can have a solar coolhouse garden on their yard and enjoy the coolness as well as the greenery and fresh fragrances coming from the horticulture.

As new developments in PV technology give rise to transparent PV cells with high efficiencies, there will not be sufficient solar radiation and light left to heat the water as well as to provide sufficient sunlight to the plants. In such cases, auxiliary solar water heaters and solar tubes for light may be utilized.

Large solar coolhouses can be turned into solar coolfarms by using translucent fabric for their roofs currently used for airports in deserts along with transparent solar photovoltaic panels and solar water heaters described here.

Benefits:

Solar coolhouses turn desert-like areas into productive and high value farming enterprises. They harness the sun’s energy and utilize most of it in the process. Besides agricultural products like vegetables, fruits, and flowers, they can also yield electrical energy, fresh water, and cool environment. By covering vast areas of the desert, they can prevent further soil loss from the deserts. They utilize seawater and local brackish water in the process and turn it into fresh water for the crops and any excess is provided to the local communities. They turn urban penthouses into roof gardens with similar products as well as reduce heat load on the surroundings.

Figures:

Figure 1 shows the solar radiation spectrum.

Figure 2 shows the rate of photosynthesis and the amounts of sunlight in the various wavelengths that are absorbed by the chlorophyll in plants.

Figure 3 shows Spectral Response of solar cells.

Figure 4 shows Spectral response of conventional and transparent PV cells.

Figure 5 shows a sample of a commercial Thin Film solar PV in fabric form. (reference available on request.)

Figure 6 shows how stacking takes advantage of the solar radiation spectrum and the spectral response of photovoltaic panels.

Figure 7 shows how the wavelength spectra can be matched so that the solar radiation can be appropriately shared and utilized by the series of devices.