ASHRAE Journal - September 2009 - 32

able to the concentrating solar renewable technologies discussed below. A few available solar conversion technologies are discussed later to identify the range of conversion efficiencies. The intent of the discussion is not to comment on the cost implication, the suitability of the technology for a particular location or application or the finer details of what the system components should be to be feasible. It is meant as a general guidance towards practical limits for conversion efficiency. Photovoltaic (PV) Power Production Annual Radiation Received (kWh/m2) Solar Collector Flat Plate, Horizontal Flat Plate, 24° Tilt Flat Plate, Horizontal, Direct Beam Radiation Flat Plate, 24° Tilt, Direct Beam Radiation One Axis Tracking 2,802 2,934 2,088 2,199 No Tracking 2,204 2,364 1,590 1,723 Table 2: Available solar radiation on collector surface (100% efficient) from calculations. The conversion efficiency of the available solar energy into useful thermal or electric energy varies greatly between different commercially available technologies (Figure 4, for example, compares PV technologies4). Solar electricity production using multicrystalline silicone photovoltaic cells typically have efficiencies of between 16% and 20%; thin film technology, sometimes a more economical choice, may deliver efficiencies of around 10%. Our approach here is focused on an early assessment of the likelihood of achieving net zero energy, regardless of cost but using available technology. For that reason a photovoltaic cell efficiency Figure 4: Photovoltaic cell conversion efficiencies improvement from 1975 through 2003.4 of 17% was chosen. The power production of these cells also deteriorates with increased outside efficiency for large scale systems (solar radiation to electrictemperature. The power degradation temperature coefficients for ity) is 17% and 31% of parabolic trough and Stirling energy these cells are often quoted as approximately –0.3% to –0.4% systems, respectively. per degree Kelvin. The actual surface temperature of the cell, Parabolic trough solar collector systems can be roof mounted that affects the conversion efficiency depends on the incident and connected to a trigeneration power generation system radiation, the local dry-bulb temperature and local wind speed, that converts high pressure steam to electricity using a steam as well as wind exposure of the panels.5 turbine generator (Figure 5). The waste heat from the turbine Using local meteorological data for Abu Dhabi and a (lower pressure hot steam) can be used in an absorption chiller predicted cell temperature based on these local conditions,5 system to produce chilled water with the remained of the waste the average panel collection efficiency may be reduced from heat (low pressure hot condensate) can be used for domestic 17% to 15%. For our calculation purposes a photovoltaic cell hot-water production before being sent back to the collector efficiency of 15% was used. A large portion of the roof area array for reheating. These systems require tracking devices can be covered by photovoltaic cells with limited space made that continuously track the sun to focus the radiation on the available for access for maintenance. For the present analysis collector absorber tube. a roof coverage factor of 75% was assumed. Considering that high pressure steam can be produced at a Other photovoltaic modules are available with higher ef- temperature of 608°F (320°C) and that the return temperature of ficiencies, such as concentrating photovoltaic that use Fresnel the steam to the collector array would be approximately 212°F lenses to concentrate incident radiation on smaller very high (100°C), a Carnot efficiency of approximately around 37% is efficiency cells. These types of emerging technology devices expected. Actual steam turbine efficiency when coupled with are not considered for the present analysis. a waste heat absorption system may be 14%. Electricity can also be produced through a solar thermal Assuming that the thermal system losses and solar absorption cycle such as parabolic trough solar collector systems (pro- (reflectivity) losses amount to 25% of the incident solar energy ducing heated oil or steam) or concentrating mirrors focusing and that the absorption chiller system has a coefficient of perradiation on a Stirling engine. Typically, the energy conversion formance (COP) of 1.4 and requires 20% of its rated power in 32 ASHRAE Journal ashrae.org September 2009

ASHRAE Journal - September 2009

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