Solar cooling systems

INTRODUCTION In solar cooling systems, solar heat is used to drive the cooling process. Thermally driven cooling machines, such as absorption- or ad-sorption chillers have been used for decades, but have been powered mainly by industrial waste heat or by district heat.

In recent years, demonstration projects have shown the potential to use solar thermal energy to drive those chillers. Because most of the available thermal chillers have large cooling capacities (often several hundred kW), the focus of R&D was largely on developing smaller cooling units as well as to improve the system design. About 100 systems have been installed in Europe and several companies are offering solar cooling solutions as part of their product and service portfolio.

Why solar cooling? Dramatic increase of air conditioning since the early 80’s Cost of energy Issues related to environmental pollution Due to energy production Due to the use of CFC’s and HCFC’s Matches demand with source availability Crucial for improving life standards in developing countries

Solar cooling systems In solar cooling systems, solar heat is used to drive the cooling process. Thermally driven cooling machines, such as absorption- or ad-sorption chillers have been used for decades, but have been powered mainly by industrial waste heat or by district heat.

Components of a solar cooling systems A solar cooling installation consists of a typical solar thermal system made up of Solar collectors Storage tank Control unit Pipes and pumps Thermally driven cooling machine-chillers

Solar Collectors Solar collectors are the key component of active solar- heating and cooling systems. They gather the sun's energy, transform its radiation into heat, then transfer that heat to a fluid (usually water or air). The solar thermal energy can be used in solar water-heating systems, solar cooling systems,solar pool heaters, and solar space-heating systems .

classification Flat-plate collectors – the absorbing surface is approximately as large as the overall collector area that intercepts the sun's rays. Concentrating collectors – large areas of mirrors or lenses focus the sunlight onto a smaller absorber.

Chillers Chillers are the core of solar cooling plants. If solar panels provide necessary energy input to the plant, chillers are those machines that are able to produce cooling by utilizing the hot water coming from solar panels.

chiller A chiller is a machine that removes heat from a liquid via a vapor-compression or absorption refrigeration cycle. Most often water is chilled, but this water may also contain ~20% glycol and corrosion inhibitors. Other fluids such as thin oils can be chilled as well.

Types of chillers There are different types of chillers: absorption type adsorption type They been used for decades but have been powered mainly by electric motors, steam or gas turbines.

Adsorption chillers

How does the absorption chiller works? Absorption chillers use solar heat energy instead of mechanical energy to provide cooling. A thermal compressor consists of an absorber, a generator, a pump, and a throttling device, and replaces the mechanical vapor compressor.

Continue…. refrigerant vapor from the evaporator is absorbed by a solution mixture in the absorber. This solution is then pumped to the generator. There the refrigerant re-vaporizes using a waste steam heat source. The refrigerant-depleted solution then returns to the absorber via a throttling device.

Absorbents commonly used The two most common refrigerant/ absorbent mixtures used in absorption chillers are water/lithium bromide. ammonia/water.

Schematic diagram of single effect solar absorption chiller- air conditioning system

Remarks absorption chillers have a low coefficient of performance (COP = chiller load/heat input). Cop is around 0.3-0.4 only absorption chillers can substantially reduce operating costs because they are powered by low-grade waste heat.

Adsorption chillers

What is adsorption chillers? Adsorption chillers apply solid sorption materials instead of a liquid solution. Market available systems use water as a refrigerant and silica gel as a sorption material.

Components of adsorption refrigerator Solar collector, adsorber/generator module/s, condenser and evaporator

Working procedure Step 1: Desorption – Drying of the adsorbent Step 2: Adsorption – water vapour is adsorbed at the surface of the adsorbent Step 3: Return of condensate

Working process of the adsorption chiller

Remarks Under typical operation conditions, with a driving heat temperature of about 80°C,adsorption chillers can achieve a COP of about 0.6, but operation is possible even at heat source temperatures of approx. 60°C. The capacity of these chillers ranges from 50 kW to 500 kW chilling power.

Other solar cooling applications SOLID DESICCANT COOLING WITH ROTATING WHEELS

What is desiccant cooling systems? Desiccant cooling systems combine sorptive dehumidification, heat recovery, evaporation and heating to create a cooling process which can offer energy savings compared to conventional air conditioning systems. Waste heat or solar energy can be used for the required regeneration of the sorbens in the dehumidifier, leading to further energy savings.

Continue…. Desiccant cooling systems (DEC) are basically open cycle systems, using water as a refrigerant in direct contact with air. The thermally driven cooling cycle is a combination of evaporative cooling with air dehumidification by a desiccant.

Adavantages of desiccant cooling systems Cost effective systems Low energy consumption Continuosly repeating cycle Commonly used dessiccants are Silica gel zeolite

Comparison of the main sorption and desiccant technologies

solar cooling applications

Wine store cooling Use: cooling of a wine store Site: Banyuls (south France) Solar thermal collector field: 130 m² evacuated tube collectors Chiller: absorption chiller with 52 kW cooling capacity

Air-conditioning of a factory Use: air conditioning of the production facilities in a cosmetics factory Site: Inofita Viotias (appr. 50 km north-east of Athens) Solar thermal collector field: 2700 m² flat plate collectors Chillers: 2 adsorptions chillers with 350 kW each 3 compression chillers with 350 kW each

Hotel air-conditioning Use: air-conditioning of a hotel and steam supply for the hotel laundry Site: Dalaman (Mediterranean coast Turkey) Solar thermal collector field: 180 m² parabolic through collectors (aperture area) Chiller: double effect absorption chiller (cooling capacity 116 kW, 4 bar)

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