Service hot water Sunlight sensor Controller Hot water to existing system Bypass valve Auxiliary conventional water heater (pressurized) Temperature sensor Parabolic troughs Hot water storage tank (non-pressurized) Solar pump Cold water supply 02279808m Figure 7. Typical parabolic-trough solar water heating system. Alternatively, a facility may pay for initial system installation itself using tradi- tional appropriations and financial analysis and project budgeting.
(See Federal Life- Cycle Costing Procedures in Appendix D.) This brings the full energy cost savings achieved back to the facility rather than splitting it with an ESCO during the con- tract term, thus generating greater savings over the project life. For most Federal facili- ties, however, ESPC is also very attractive financially—especially for capital-intensive projects such as parabolic-trough systems. Private companies can often take advantage of tax credits for renewable energy or accel- erated depreciation of the capital investment that the facility itself cannot.
Thus, many projects for Federal facilities that do not meet the 10-year simple payback criterion that mandates construction, or the positive life-cycle cost analysis that allows construc- tion, could be attractive investments for private companies. IST has, in fact, used third-party financing for all of its projects thus far. The investor pays IST to build and maintain the systems and then collects the energy-savings payments from the facility. water that will actually be used. (See Figure 7).
Parabolic-trough collectors gener- ally require more supervision and mainte- nance than nonconcentrating solar collectors—the reflectors must be cleaned and checked for leaks two to four times per year. These systems particularly benefit from economies of scale, so are generally used for larger systems. Parabolic-trough solar collectors use curved mirrors to focus sunlight on a receiver tube running through the focal line of the mirrors. The pressurized tube through which the antifreeze solution circulates is coated with a special heat-absorbent surface and encased in a glass tube.
IST parabolic- trough collectors can heat their transfer fluid to as much as 520˚F (271˚C) or more, but for domestic water supply systems, they Industrial Solar Technology/PIX05641 Energy-Saving Mechanism Parabolic-trough collectors are typically used in active, indirect water-heating sys- tems. Such systems use pumps to circulate an antifreeze solution between the collectors and the storage tank, where a heat exchanger transfers heat from the circulating fluid to water in the thermal storage tank. Potable water flows through a second heat exchanger in the storage tank, absorbing heat for the Figure 8.
Parabolic-trough collector systems also work well for central plants serving district heating systems. This system at the California Correctional Institution at Tehachapi augments a central boiler that provides several buildings with hot water for space heating as well as for domestic use. District systems such as this one will generally not include storage—contributing only when they are actually operating. They will therefore meet a smaller portion of the load than the 50% to 60% typical for systems including storage. 10