tor system, together with a 5000-gallon (19,000-liter), nonpressurized, stainless- steel thermal storage tank, on a payment- for-energy-savings basis. IST expanded the system to 7200 square feet (669 square meters) of collector in 1988 in response to increased hot-water use resulting from increased inmate population. The parabolic- trough system preheats the water supply to five of the six boilers and thereby provides about 40% (1400 million of 3500 MBtus per year) of the energy required by the prison for water heating. The parabolic troughs heat the transfer fluid (ethylene glycol mixture) to as much as 256˚F (124˚C).
Whenever the amount of direct sunlight is above a set threshold, the circulating pump turns on, sending the heated antifreeze mixture through a copper- coil heat exchanger in the storage tank, heating that water to as much as 185˚F (85˚C). The system has operated continuously since 1987 except for a 2-month period after the line from the collector field to the facil- ity was broken by a backhoe during a con- struction project. Deep snows occasionally keep the tracking system from working and the system automatically shuts down during high winds. IST personnel clean the reflec- tors and check the system for leaks two or three times per year.
The system's operation has little impact on facility personnel who know that it is there and how to turn it off if anything should ever go wrong. Otherwise they have nothing to do with it. The Adams County Detention Facility system is owned and operated by third-party investors. The investors contracted with IST to design and construct the system. Initial cost of the system was $218,400 but a $40,000 state "buy-down" subsidy reduced the price to $178,400 and the investors ben- efitted $21,840 from a 10% tax credit. The owners are responsible (with guidance from IST) for major repairs or renovations and pay IST to operate and maintain the system.
Adams County pays the owners for the heat provided by the system—based on readings from a Btu meter at the facility—at a dis- count from the current rate they pay for nat- ural gas. The county spends about $200 per month less for the combination of the trough system plus utility bills than it would with- out the solar collector system. The system has been operating for more than 10 years. Because the facility's backup conventional boilers are 70% efficient, the Warren Gretz, NREL/PIX00327 Figure 13. Parabolic-trough solar water-heating system for Adams County Detention Facility, Brighton, Colorado.
1400 MBtus per year of solar energy pro- vided by the parabolic-trough system saves 2000 MBtus of gas per year—enough energy for total energy consumption by more than 20 households. The total reduction in natural gas consumption thus far is about 20 billion Btus. Over the life of the project, approximately 50 billion fewer Btus of natural gas (equivalent to about 370,000 gallons of fuel oil) will be burned, with associated savings in pollution and greenhouse gas emission. As can be seen from the added notes to the life-cycle cost calculations in Appendix E, the Adams County system would not have been economically viable at the full cost of $218,400.
At the state-subsidized cost of $178,400, however, the parabolic- trough system yields discounted cost sav- ings of about $29,000 over the life of the project. As can be seen from the cash flow calculation in Appendix F, however, the tax benefits of 10% investment credit plus accelerated depreciation that are available to the private investors who funded the project provide substantial additional savings. The investors are able to pass on about $200 per month savings to the facility and still get a 6% return on their investment, despite lower than anticipated natural gas costs.
The county has already saved nearly $24,000; projected savings for the 25-year life of the project, based on actual natural gas costs for the first 10 years and 3% escalation for the future, are $58,000. The 10-year operating history of the par- abolic-trough solar water-heating system for the Adams County Detention Facility shows clearly that the technology works, and that with installation on a payment-for-energy- savings basis, operation posed no burden on the facility or on facility staff. The cost cal- culations for the project show that subsidies can provide the difference to make a project economically viable and that the tax benefits to a private funder can be very important.
The project's economic history and less- favorable-than-anticipated return for the investors also shows that one cannot count too heavily on conventional energy prices rising (something not done under the IDIQ). From the facility's perspective, however, this reinforces the fact that payment for savings is a guaranteed-win situation, regardless of whether energy prices rise or fall. Case Study 2 — Federal Correctional Institution, Phoenix, Arizona Located about 10 miles (16 kilometers) north of Phoenix, Arizona, the Federal Correctional Institution (FCI-Phoenix) is a 18