Page 263 - Alternative Energy Systems in Building Design

SOLAR COOLING AND AIR–CONDITIONING 239

TABLE 6.2 TYPICAL BUILDING COOLING CAPACITIES

SPACE SIZE COOLING TONS
Medium office 50,000 100–150
Hospital 150,000 400–600
Hotel 250,000 400–500
High school 50,000 100–400
Retail store 160,000 170–400

subsequent cycle, energy is removed from the liquid in a form of evaporation or gas
expansion that disperses the gas molecules and turns the surrounding chamber into a
cold environment. Table 6.2 outlines air-conditioning cooling capacity requirement for
various types of buildings.
A medium of energy-absorbing liquid, such as water or air, when circulated within
the so-called evaporation chamber, gives up its heat energy to the expanded gas. The
cold water or air, in turn, is circulated by means of pumps into environments that have
higher ambient heat-energy levels. The circulated cold air, in turn, is passed into the
ambient space through radiator tubes or fins, thus lowering the energy of the envi-
ronment. Temperature control is realized by the opening and closing of cold medium
circulating tube valves or air-duct control vanes, modulated by a local temperature-
sensing device such as a thermostat or a setpoint-control mechanism.

COOLING TECHNOLOGIES
There are two types of refrigeration technologies currently in use, namely, electric
vapor compression (Freon gas) and heat-driven absorption cooling. Absorption-
cooling chillers are operated by steam, hot water, fossil-fuel burners, or combinations
of these. There are two types of absorption chillers. One uses lithium bromide (LiBr)
as an energy-conversion medium and water as a refrigerant. In this type of technology,
the lowest temperature achieved is limited to 40°F. Another absorption-chiller tech-
nology uses ammonia as the energy-conversion medium and a mix of ammonia and
water as the refrigerant. The lowest temperature limit for this technology is 20°F. Both
these technologies have been around for about 100 years.
The basic principle of absorption chillers is gasification of LiBr or ammonia.
Gasification takes place when either of the media is exposed to heat. Heat could be
derived from fossil-fuel burners, hot water obtained from geothermal energy, passive
solar water heaters, or microturbine generators, which use landfill gases to produce
electricity and heat energy.

COEFFICIENT OF PERFORMANCE (COP)
The energy efficiency of an air-conditioning system is defined by a coefficient of
performance (COP), which is the ratio of cooling energy to the energy supplied to the