Direct Solar Energy
otherwise be viable, but which are superior to conventional heating systems in many respects. Better-insulated houses eliminate moisture problems associated, for example, with thermal bridges and damp basements. Increased roof insulation also increases the attenuation of outside sounds such as from aircraft.
For active solar heating and cooling, the environmental impact of solar water-heating schemes in the UK would be very small according to Boyle (1996). For example, in the UK, the materials used are those of every- day building and plumbing. Solar collectors are installed to be almost indistinguishable visually from normal roof lights. In Mediterranean countries, the use of free-standing thermosyphon systems on flat roofs can be visually intrusive. However, the collector is not the problem, but rather, the storage tank above it. A study of the lifecycle environmental impact of a thermosyphon domestic solar hot water system in compar- ison with electrical and gas water heating shows that these systems have improved LCA indices over electrical heaters, but the net gain is reduced by a factor of four when the primary energy source is natural gas instead of electricity (Tsilingiridis et al., 2004).
With regard to complete solar domestic hot water systems, the energy payback time requires accounting for any difference in the size of the hot water storage tank compared to the non-solar system and the energy used to manufacture the tank (Harvey, 2006). It is reported that the energy payback time for a solar/gas system in southern Australia is 2 to 2.5 years, despite the embodied energy being 12 times that of a tank- less system. For an integrated thermosyphon flat-plate solar collector and storage device operating in Palermo (Italy), a payback time of 1.3 to 4.0 years is reported (Harvey, 2006).
PV systems do not generate any type of solid, liquid or gaseous by- products when producing electricity. Also, they do not emit noise or use non-renewable resources during operation. However, two topics are often considered: 1) the emission of pollutants and the use of energy during the full lifecycle of PV manufacturing, installation, operation and maintenance (O&M) and disposal; and 2) the possibility of recycling the PV module materials when the systems are decommissioned.
Starting with the latter concern, the PV industry uses some toxic, explo- sive gases, GHGs, as well as corrosive liquids, in its production lines. The presence and amount of those materials depend strongly on the cell type (see Section 3.3.3). However, the intrinsic needs of the produc- tion process of the PV industry force the use of quite rigorous control methods that minimize the emission of potentially hazardous elements during module production.
Recycling the material in PV modules is already economically viable, mainly for concentrated and large-scale applications. Projections are that between 80 and 96% of the glass, ethylene vinyl acetate, and metals (Te, selenium and lead) will be recycled. Other metals, such as Cd, Te, tin, nickel, aluminium and Cu, should be saved or they can be recycled by other methods. For discussions of Cd, for example,
see Sinha et al. (2008), Zayed and Philippe (2009) and Wadia et al. (2009).
It is noted that, in certain locations, periodic cleaning of the PV panels may be necessary to maintain performance, resulting in non- negligible water requirements.
With respect to lifecycle GHG emissions, Figure 3.14 shows the result of a comprehensive literature review of PV-related lifecycle assess- ment (LCA) studies published since 1980 conducted by the National Renewable Energy Laboratory. The majority of lifecycle GHG emis- sion estimates cluster between about 30 and 80 g CO2eq/kWh, with potentially important outliers at greater values (Figure 3.14). Note that the distributions shown in Figure 3.14 do not represent an assessment of likelihood; the figure simply reports the distribution of currently published literature estimates passing screens for qual- ity and relevance. Refer to Annex II for a description of literature search methods and complete reference list, and Section 126.96.36.199 for further details on interpretation of LCA data. Variability in esti- mates stems from differences in study context (e.g., solar resource, technological vintage), technological performance (e.g., efficiency, silicon thickness) and methods (e.g., LCA system boundaries). Efforts to harmonize the methods and assumptions of these studies are recommended such that more robust estimates of central tendency and variability can be realized, as well as a better understanding of the upper-quartile estimates. Further LCA studies are also needed to increase the number of estimates for some technologies (e.g., CdTe).
As for the energy payback of PV (see also Box 9.3), Perpinan et al. (2009) report paybacks of 2.0 and 2.5 years for microcrystalline sili- con and monocrystalline silicon PV, respectively, taking into account use in locations with moderate solar irradiation levels of around 1,700 kWh/m2/yr (6,120 MJ/m2/yr). Fthenakis and Kim (2010) show payback times of grid-connected PV systems that range from 2 to 5 years for locations with global irradiation ranges from 1,900 to 1,400 kWh/m2/yr (6,840 MJ/m2/yr).
For CSP plants, the environmental consequences vary depending on the technology. In general, GHG emissions and other pollutants are reduced without incurring additional environmental risks. Each square metre of CSP concentrator surface is enough to avoid the annual production of 0.25 to 0.4 t of CO2. The energy payback time of CSP systems can be as low as five months, which compares very favourably with their lifespan of about 25 to 30 years (see Box 9.3 for further discussion). Most CSP solar field materials can be recycled and reused in new plants (SolarPACES, 2008).
Land consumption and impacts on local flora and wildlife during the build-up of the heliostat field and other facilities are the main environ- mental issues for CSP systems (Pregger et al., 2009). Other impacts are associated with the construction of the steel-intensive infrastructure for solar energy collection due to mineral and fossil resource consumption,