Solar air conditioning

History

In the late 19th century, the most common fluid for water absorption was a solution of ammonia and water. Today, the combination of lithium bromide and water is also in common use. One end of the system of expansion / condensation pipes is heated, and the other end gets cold enough to make ice. Originally, natural gas was used as a heat source in the late 19th century. Today, propaneis used in recreational vehicle absorption chiller refrigerators. Hot water solar thermal energy collectors can also be used as the “free energy” heat source. A National Aeronautics and Space Administration (NASA) sponsored report in 1976 surveyed solar energy system applications of air conditioning. Techniques discussed included both solar powered and heat related (Rankine cycle) and solar energy (heat pump) along with an extensive bibliography of related literature. ^[2]

Photovoltaic (PV) solar cooling

Photovoltaics can provide the power of the conventional compressor-based adsorption / absorption-based, but 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 it does not include the principles of solar energy. cooling ).

Since PV cooling has cost effectively, it has had a significant impact on the efficiency of the environment. Using more efficient electrical cooling methods and permits.

For example, a 100,000 BTU US Energy Star rated ^{[rating 1]} 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 a 20 kW solar photovoltaic electricity generation system with storage.

A solar-tracking 7 kW photovoltaic system would probably have a price of over $ 20,000 USD (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 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 be found in the range of 20 (±). A 100,000 BTU SEER 20 air conditioner would require less than 5 kW while operating.

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 US) indirect evaporative coolers can achieve a SEER above 20, and up to SEER 40. At 100,000 indirect BTU 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 power of air conditioning (and other uses). With American state government subsidies of $ 2.50 to $ 5.00 USD per photovoltaic watt, ^[3] the amortized cost of PV-generated electricity can be below $ 0.15 per kWh. This is currently $ 0.15 or more. Excess PV power generated when it is not necessary to reduce the power grid in many locations, which can reduce (or eliminate) annual net electricity purchase requirement. (See Zero-energy building )

Superior energy efficiency can be designed into new construction (or retrofitted to existing buildings). Since the US Department of Energy was created in 1977, their Weatherization Assistance Program ^[4] has reduced heating-and-cooling load on 5.5 million low-income affordable homes by an average of 31 percent. A hundred million American buildings still need improved weatherization. Careless construction practices are still producing inefficient new buildings.

It is fairly simple to reduce the heating-and-cooling requirement for new construction by one half. This can not be done without additional cost, because there are costs for other air conditioning systems and other benefits.

Geothermal cooling

Earth sheltering or earth cooling tubes can take advantage of the ambient temperature of the earth to reduce or In many climates where the majority of humans live, they can greatly reduce the buildup of heat, and also help remove heat from the interior of the building. They increase construction cost, but reduce the cost of the conventional air conditioning equipment.

Earth cooling tubes are not cost effective in hot humid environments where the ambient temperature. A solar chimney or photovoltaic- powered fan can be used to exhaust heat and draw in cooler, dehumidified air that has passed by ambient Earth temperature surfaces. Control of humidity and condensation are important design issues.

A geothermal heat pump uses an ambient temperature to improve SEER for heat and cooling. A deep well recirculated water to Earth temperature extract (typically at 2 gallons of water per ton per minute). These “open loop” systems were the most common in early life systems, but could not be better. Another method is a closed loop system, in which a loop of tubing is in progress, or in trenches in the law, to cool an intermediate fluid. When they are used, they are back-filled with Bentonite or another material to ensure good thermal conductivity to the earth.

It is a mixture of propylene glycol because it is non-toxic unlike ethylene glycol (which is used in car radiators). Propylene glycol is viscous, and would eventually be part of the loop (s), so it has fallen out of favor. Today, the most common transfer agent is a mixture of water and ethyl alcohol (ethanol).

Ambient earth temperature is much lower than peak summer air temperature, and much higher than the lowest extreme winter air temperature. Water is 25 times more thermally conductive than air, so it is much more efficient than an outside air heat pump, (which becomes less effective when the outside temperature drops in Winter).

The same type of geothermal can be used without a heat pump but greatly diminished results. Ambient Earth temperature is pumped through a shrouded radiator (like an automobile radiator). Air is blown across the radiator, which cools without a compressor-based air conditioner. Photovoltaic solar electric panels produce electricity for the water pump and fan, eliminating air-conditioning utility bills. This concept is cost-effective, as long as the location has the environment temperature below the thermal comfort zone (not the tropics).

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 dehumidify, in a cost-effective, low-energy-consumption, continuously repeating cycle. ^[5] 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 provides a controlled way of ventilation while minimizing energy loss. Air is passed through an ” enthalpy wheel ” (often using silica gel) to reduce the cost of heating air into the winter by supplying air to the air. In the summer, the inside air cools the warmer ^[6] This low-energy fan-and-motor ventilation system can be cost-effectively powered by photovoltaics , with enhanced natural convection exhausting 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. ^[7]

Solar solar thermal collectors provide input energy for a cooling system. There are several commercially available systems that blow air through a depressant 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 liquid and liquid desiccant but are not normally used commercially available machines. Preheating of the air is shown to greatly enhance desiccant regeneration. The packed column yields a dehumidifier / regenerator, provided with a reduced pressure. ^[8]

Passive solar cooling

In this type of cooling solar thermal energy is not directly applied to a cold environment or 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 lathing Primarily from the sun.

For example, a sign of poor thermal design is an attic that gets hotter in summer than the outside temperature. This Can Be Significantly Reduced or eliminated with a cool roof gold green roof , qui can Reduce the roof surface temperature by 70 ° F (40 ° C) in summer. A radiant barrier on the roof of 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 buildingin a hot climate.

Solar closed-loop absorption cooling

The following are common technologies for solar thermal closed-loop air conditioning.

• Absorption: NH
  3 / H
  2 O or Ammonia / Water
• Absorption: Water / Lithium Bromide
• Absorption: Water / Lithium Chloride
• Adsorption: Water / Silica Gel Gold Water / Zeolite
• Adsorption: Methanol / Activated Carbon ^[9]

Active solar cooling uses solar thermal collectors to provide solar energy to thermally driven chillers (usually adsorption or absorption chillers). ^[10] 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 is used for large commercial and industrial cooling.

Solar thermal energy can be used to effectively 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. ^[11]

Efficient absorption chillers nominally require water at least 190 ° F (88 ° C). Common, inexpensive flat- hot thermal collectors only produce about 160 ° F (71 ° C) water. High temperature flat plate, concentrating (CSP) or evacuated tube collectors are needed to produce the higher temperature transfer fluids required. In large scale, there are several successful projects both for technical and economic operations worldwide, for example, at the headquarters of Caixa Geral de Depósitos in Lisbon with 1,579 square meters (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 they have shown that they can be used for temperatures over 200 ° F (93 ° C) (including double glazing, increased backside insulation, etc.). ^[12] Where 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 of a solar air conditioning facility, ^[13] [14] . ^{[ citation needed ]} The Southern California Gas Co. (The Gas Company) is also testing the practice of solar thermal cooling systems at their Energy Resource Center (ERC) in Downey, California. Solar collectors from Sopogy and Cogenra have been installed on the rooftop at the ERC and are producing cooling for the building’s air conditioning system. ^[15] Masdar City in the United Arab Emiratesis also testing a double-effect absorption cooling plant using Sopogy parabolic trough collectors, ^[16] Mirroxx Fresnel array and TVP Solar high-vacuum solar thermal panels. ^[17]

For 150 years, absorption chillers have been used to make ice (before the electric light bulbs were invented). ^[18] 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 New Otani Hotel Tokyo in Japan. ^[19] Mathematical models are available in the public domain for ice-based thermal energy storage performance calculations. ^[20]

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. ^[21]

Providers of solar cooling systems include ChillSolar, ^[22] SOLID, ^[23] Sopogy , ^[24] Cogenra, ^[25] Mirroxx ^[26] and TVP Solar ^[27] for commercial installations and ClimateWell , ^[28] 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. ^[29]

Zero-energy buildings

Goals of zero-energy buildings include sustainable , green building technologies that can significantly reduce, or eliminate, net annual energy bills. The supreme achievement is the totally off-the-grid autonomous building That does not-have to be connected to utility companies. In hot climates with significant degree days of cooling demand, the leading-edge solar air conditioning will be an additional important critical success factor .

See also

• Passive house
• Passive solar building design
• Solar powered refrigerator

Notes

References