Solar Refrigeration
A solar-powered refrigerator is a refrigerator which runs on energy directly provided by sun, and may include photovoltaic or solar thermal energy.
Solar-powered refrigerators are able to keep perishable goods such as meat and dairy cool in hot climates, and are used to keep much needed vaccines at their appropriate temperature to avoid spoilage.
Solar-powered refrigerators may be most commonly used in the developing world to help mitigate poverty and climate change.
Traditionally solar-powered refrigerators and vaccine coolers use a combination of solar panels and lead batteries to store energy for cloudy days and at night in the absence of sunlight to keep their contents cool. These fridges are expensive and require heavy lead-acid batteries which tend to deteriorate, especially in hot climates, or are misused for other purposes. In addition, the batteries require maintenance, must be replaced approximately every three years, and must be disposed of as hazardous wastes possibly resulting in lead pollution. These problems and the resulting higher costs have been an obstacle for the use of solar powered refrigerators in developing areas.
In the mid-1990s NASA JSC began work on a solar powered refrigerator that used phase change material rather than battery to store “thermal energy” rather than “chemical energy.” The resulting technology has been commercialized and is being used for storing food products and vaccines.
Solar Air Conditioning
Solar air conditioning refers to any air conditioning (cooling) system that uses solar power.
This can be done through passive solar, solar thermal energy conversion and photovoltaic conversion (sunlight to electricity). The U.S. Energy Independence and Security Act of 2007 created 2008 through 2012 funding for a new solar air conditioning research and development program, which should develop and demonstrate multiple new technology innovations and mass production economies of scale. Solar air conditioning might play an increasing role in zero-energy and energy-plus buildings design.
Photovoltaic (PV) solar cooling – Photovoltaics can provide the power for any type of electrically powered cooling be it conventional compressor-based or adsorption/absorption-based, though the most common implementation is with compressors. For small residential and small commercial cooling (less than 5 MWh/a) PV-powered cooling has been the most frequently implemented solar cooling technology. The reason for this is debated, but commonly suggested reasons include incentive structuring, lack of residential-sized equipment for other solar-cooling technologies, the advent of more efficient electrical coolers, or ease of installation compared to other solar-cooling technologies (like radiant cooling).
Since PV cooling’s cost effectiveness depends largely on the cooling equipment and given the poor efficiencies in electrical cooling methods until recently it has not been cost effective without subsidies. Using more efficient electrical cooling methods and allowing longer payback schedules is changing that scenario.
For example, a 100,000 BTU U.S. Energy Star rated air conditioner with a high seasonal energy efficiency ratio (SEER) of 14 requires around 7 kW of electric power for full cooling output on a hot day. This would require over a 7 kW solar photovoltaic electricity generation system.
A solar-tracking 7 kW photovoltaic system would probably have an installed price well over $20,000 USD (with PV equipment prices currently falling at roughly 17% per year). Infrastructure, wiring, mounting, and NEC code costs may add up to an additional cost; for instance a 3120 watt solar panel grid tie system has a panel cost of $0.99/watt peak, but still costs ~$2.2/watt hour peak. Other systems of different capacity cost even more, let alone battery backup systems, which cost even more.
A more efficient air conditioning system would require a smaller, less-expensive photovoltaic system. A high-quality geothermal heat pump installation can have a SEER in the range of 20 (±). A 100,000 BTU SEER 20 air conditioner would require less than 5 kW while operating. Newer and lower power technology including reverse inverter DC heat pumps can achieve SEER ratings up to 26.
There are new non-compressor-based electrical air conditioning systems with a SEER above 20 coming on the market. New versions of phase-change indirect evaporative coolers use nothing but a fan and a supply of water to cool buildings without adding extra interior humidity (such as at McCarran Airport Las Vegas Nevada). In dry arid climates with relative humidity below 45% (about 40% of the continental U.S.) indirect evaporative coolers can achieve a SEER above 20, and up to SEER 40. A 100,000 BTU indirect evaporative cooler would only need enough photovoltaic power for the circulation fan (plus a water supply).
A less-expensive partial-power photovoltaic system can reduce (but not eliminate) the monthly amount of electricity purchased from the power grid for air conditioning (and other uses). With American state government subsidies of $2.50 to $5.00 USD per photovoltaic watt, the amortized cost of PV-generated electricity can be below $0.15 per kWh. This is currently cost effective in some areas where power company electricity is now $0.15 or more. Excess PV power generated when air conditioning is not required can be sold to the power grid in many locations, which can reduce (or eliminate) annual net electricity purchase requirement.
Superior energy efficiency can be designed into new construction (or retrofitted to existing buildings). Since the U.S. Department of Energy was created in 1977, their Weatherization Assistance Program has reduced heating-and-cooling load on 5.5 million low-income affordable homes an average of 31%. A hundred million American buildings still need improved weatherization. Careless conventional construction practices are still producing inefficient new buildings that need weatherization when they are first occupied.
It is fairly simple to reduce the heating-and-cooling requirement for new construction by one half. This can often be done at no additional net cost, since there are cost savings for smaller air conditioning systems and other benefits.
Solar Thermal Compression Technology – Everyone who understands refrigeration knows that wherever we have gas; the ideal gas law applies. In the refrigeration process, refrigerant is in gaseous state from when it leaves the evaporator through to the condenser.
Therefore the ideal gas law applies – p * V = n * R * T
Pressure (p) of a gas multiplied with its volume (V) corresponds to its temperature (T) and the amount of the gas molecules (n) in that volume. The formula is originally based on the Avogadro constant being the same for all types of gases. However since we use a specific gas, we need to cover the different properties of that gas in a separate constant. In this case R describes the refrigerant and is, a constant. This loop is closed. The volume inside of the piping and vessels i.e. compressor, accumulators, separators etc is always constant, so the volume (V) is constant. In this case, the gas R and V are constant, so therefore the formula basically cuts down to only 3 change variables: p ~ n x T
p * V = n * R * T When the temperature of the gas changes, there are only two things that can happen, either: 1. the pressure (p) increases 2. or the number (n) of gas molecules decreases There are no other options.
The technology saves substantial amounts of energy on both fixed speed multi-stage and variable speed (inverter or digital scroll for example) compression. However, it has little if any impact on single fixed speed compression.
The solar collection system absorbs the infrared and other radiance from the sun. The vacuum tubes become internally hot and therefore heat the refrigerant gas as it passes through. In turn the gas is provided with thermal energy, this process then raises the internal energy (the sum of all microscopic kinetic and potential energy of the molecules) the molecules move with a higher value of kinetic energy which simply implies each molecule moves with a higher velocity than before, as the molecules collide with one another they rebound with higher energies, moreover intermolecular forces weaken and the molecules space out further which then results in an increase in volume. At this point most, if not all of the pressure produced from the additional heat is released in mass volume flow and therefore substantially increased molecular velocity of the refrigerant gas, thus resulting in an increase in the ΔT between condensing and ambient temperatures, effectively increasing the surface area of the condenser.
First and foremost, the systems condenser is designed to manage the compressors running at full load. The reality is when the sun is in the sky (the time when most systems run at full load), this process will ensure that all compressors virtually never run at full load. Of course it would be fantastic to achieve the additional efficiency without additional heat, but the additional heat is the price we pay in return for the higher efficiency. We actually want the heat from the solar collectors, utilising the heat (temperature not pressure) increasing the gas molecules kinetic energy, hence increasing the mass volume flow, and additionally creating an improved Delta T therefore actually enhancing the efficiency of the heat exchanger (condenser). All Refrigeration and A/C systems are manufactured to a very specific design point, e.g. for A/C systems ISO 5151. This point sets the capacities for the compressor, condenser and all other parts. The facts covered in the second paragraph above allows SolarCool to easily cope with any additional heat.
Solar open-loop Air Conditioning using desiccants – Air can be passed over common, solid desiccants (like silica gel or zeolite) or liquid desiccants (like lithium bromide/chloride) to draw moisture from the air to allow an efficient mechanical or evaporative cooling cycle. The desiccant is then regenerated by using solar thermal energy to dehumidfy, in a cost-effective, low-energy-consumption, continuously repeating cycle. A photovoltaic system can power a low-energy air circulation fan, and a motor to slowly rotate a large disk filled with desiccant.
Energy recovery ventilation systems provide a controlled way of ventilating a home while minimizing energy loss. Air is passed through an “enthalpy wheel” (often using silica gel) to reduce the cost of heating ventilated air in the winter by transferring heat from the warm inside air being exhausted to the fresh (but cold) supply air. In the summer, the inside air cools the warmer incoming supply air to reduce ventilation cooling costs. This low-energy fan-and-motor ventilation system can be cost-effectively powered by photovoltaics, with enhanced natural convection exhaust up a solar chimney – the downward incoming air flow would be forced convection (advection).
A desiccant like calcium chloride can be mixed with water to create an attractive recirculating waterfall, that dehumidifies a room using solar thermal energy to regenerate the liquid, and a PV-powered low-rate water pump
Active solar cooling wherein solar thermal collectors provide input energy for a desiccant cooling system. There are several commercially available systems that blow air through a desiccant impregnated medium for both the dehumidification and the regeneration cycle. The solar heat is one way that the regeneration cycle is powered. In theory packed towers can be used to form a counter-current flow of the air and the liquid desiccant but are not normally employed in commercially available machines. Preheating of the air is shown to greatly enhance desiccant regeneration. The packed column yields good results as a dehumidifier/regenerator, provided pressure drop can be reduced with the use of suitable packing.
Passive solar cooling – In this type of cooling solar thermal energy is not used directly to create a cold environment or drive any direct cooling processes. Instead, solar building design aims at slowing the rate of heat transfer into a building in the summer, and improving the removal of unwanted heat. It involves a good understanding of the mechanisms of heat transfer: heat conduction, convective heat transfer, and thermal radiation, the latter primarily from the sun.
For example, a sign of poor thermal design is an attic that gets hotter in summer than the peak outside air temperature. This can be significantly reduced or eliminated with a cool roof or a green roof, which can reduce the roof surface temperature by 70 °F (40 °C) in summer. A radiant barrier and an air gap below the roof will block about 97% of downward radiation from roof cladding heated by the sun.
Passive solar cooling is much easier to achieve in new construction than by adapting existing buildings. There are many design specifics involved in passive solar cooling. It is a primary element of designing a zero energy building in a hot climate.
Solar closed-loop absorption cooling – The following are common technologies in use for solar thermal closed-loop air conditioning.
- Absorption: NH3/H2O or Ammonia/Water
- Absorption: Water/Lithium Bromide
- Absorption: Water/Lithium Chloride
- Adsorption: Water/Silica Gel or Water/Zeolite
- Adsorption: Methanol/Activated Carbon
Active solar cooling uses solar thermal collectors to provide solar energy to thermally driven chillers (usually adsorption or absorption chillers). Solar energy heats a fluid that provides heat to the generator of an absorption chiller and is recirculated back to the collectors. The heat provided to the generator drives a cooling cycle that produces chilled water. The chilled water produced is used for large commercial and industrial cooling.
Solar thermal energy can be used to efficiently cool in the summer, and also heat domestic hot water and buildings in the winter. Single, double or triple iterative absorption cooling cycles are used in different solar-thermal-cooling system designs. The more cycles, the more efficient they are. Absorption chillers operate with less noise and vibration than compressor-based chillers, but their capital costs are relatively high.
Efficient absorption chillers nominally require water of at least 190 °F (88 °C). Common, inexpensive flat-plate solar thermal collectors only produce about 160 °F (71 °C) water. High temperature flat plate, concentrating or evacuated tube collectors are needed to produce the higher temperature water required. In large scale installations there are several projects successful both technical and economical in operation world wide including, for example, at the headquarters of Caixa Geral de Depósitos in Lisbon with 1,579 square metres (17,000 sq ft) solar collectors and 545 kW cooling power or on the Olympic Sailing Village in Qingdao/China. In 2011 the most powerful plant at Singapore’s new constructed United World College will be commissioned (1500 kW).
These projects have shown that flat plate solar collectors specially developed for temperatures over 200 °F (93 °C) (featuring double glazing, increased backside insulation, etc.) can be effective and cost efficient. Where water can be heated well above 190 °F (88 °C), it can be stored and used when the sun is not shining.
The Audubon Environmental Center at the Ernest E. Debs Regional Park in Los Angeles has an example solar air conditioning installation, which failed fairly soon after commissioning and is no longer being maintained. The Southern California Gas Co. (The Gas Company) is also testing the practicality of solar thermal cooling systems at their Energy Resource Center (ERC) in Downey, California. Solar Collectors from Sopogy and Cogenra were installed on the rooftop at the ERC and are producing cooling for the building’s air conditioning system. Masdar City in the United Arab Emirates is also testing a double-effect absorption cooling plant using Sopogy parabolic trough collectors, Mirroxx Fresnel array and TVP Solar high-vacuum solar thermal panels.
For 150 years, absorption chillers have been used to make ice (before the electric light bulbs were invented). This ice can be stored and used as an “ice battery” for cooling when the sun is not shining, as it was in the 1995 Hotel New Otani Tokyo in Japan. Mathematical models are available in the public domain for ice-based thermal energy storage performance calculations.
The ISAAC Solar Icemaker is an intermittent solar ammonia-water absorption cycle. The ISAAC uses a parabolic trough solar collector and a compact and efficient design to produce ice with no fuel or electric input, and with no moving parts.
Providers of solar cooling systems include SOLID, Sopogy, Cogenra, Mirroxx and TVP Solar for commercial installations and ClimateWell, Fagor-Rotartica, SorTech and Daikin mostly for residential systems. Cogenra uses solar co-generation to produce both thermal and electric energy that can be used for cooling.
Solar Greenhouses
A greenhouse (also called a glasshouse or a hothouse) is a building or complex in which plants are grown. These structures range in size from small sheds to industrial-sized buildings. A miniature greenhouse is known as a cold frame.
Commercial glass greenhouses are often high tech production facilities for vegetables or flowers. The glass greenhouses are filled with equipment like screening installations, heating, cooling, lighting and also may be automatically controlled by a computer to maximize potential growth.
A greenhouse is a structural building with different types of covering materials, such as a glass or plastic roof and frequently glass or plastic walls; it heats up because incoming visible sunshine is absorbed inside the structure. Air warmed by the heat from warmed interior surfaces is retained in the building by the roof and wall; the air that is warmed near the ground is prevented from rising indefinitely and flowing away. This is not the same mechanism as the “greenhouse effect”.
Solar greenhouses differ from conventional greenhouses in the following four ways.(1) Solar greenhouses:
- have glazing oriented to receive maximum solar heat during the winter.
- use heat storing materials to retain solar heat.
- have large amounts of insulation where there is little or no direct sunlight.
- use glazing material and glazing installation methods that minimize heat loss.
- rely primarily on natural ventilation for summer cooling.
Understanding these basic principles of solar greenhouse design will assist you in designing, constructing, and maintaining an energy-efficient structure. You can also use these concepts to help you search for additional information, either on the “Web,” within journals, or in books at bookstores and libraries.
Solar Greenhouse Design – Attached solar greenhouses are lean-to structures that form a room jutting out from a house or barn. These structures provide space for transplants, herbs, or limited quantities of food plants. These structures typically have a passive solar design.
Freestanding solar greenhouses are large enough for the commercial production of ornamentals, vegetables, or herbs. There are two primary designs for freestanding solar greenhouses: the shed type and the hoophouse. A shed-type solar greenhouse is oriented to have its long axis running from east to west. The south-facing wall is glazed to collect the optimum amount of solar energy, while the north-facing wall is well-insulated to prevent heat loss. This orientation is in contrast to that of a conventional greenhouse, which has its roof running north-south to allow for uniform light distribution on all sides of the plants. To reduce the effects of poor light distribution in an east-west oriented greenhouse, the north wall is covered or painted with reflective material.
Solar Furnace And Applications
A solar furnace is a structure that uses concentrated solar power to produce high temperatures, usually for industry. Parabolic mirrors or heliostats concentrate light (Insolation) onto a focal point. The temperature at the focal point may reach 3,500 °C (6,330 °F), and this heat can be used to generate electricity, melt steel, make hydrogen fuel or nanomaterials.
The largest solar furnace is at Odeillo in the Pyrénées- Orientales in France, opened in 1970. It employs an array of plane mirrors to gather sunlight, reflecting it onto a larger curved mirror.
The rays are focused onto an area the size of a cooking pot and can reach 4,000 °C (7,230 °F), depending on the process installed, for example:
- about 1,000 °C (1,830 °F) for metallic receivers producing hot air for the next generation solar towers as it will be tested at the Themis plant with the Pegase project
- about 1,400 °C (2,550 °F) to produce hydrogen by cracking methane molecules
- up to 2,500 °C (4,530 °F) to test materials for extreme environment such as nuclear reactors or space vehicle atmospheric reentry
- up to 3,500 °C (6,330 °F) to produce nonmaterial’s by solar induced sublimation and controlled cooling, such as carbon nanotubes or zinc nanoparticles
It has been suggested that solar furnaces could be used in space to provide energy for manufacturing purposes.
Their reliance on sunny weather is a limiting factor as a source of renewable energy on Earth but could be tied to thermal energy storage systems for energy production through these periods and into the night.
