Solar Energy

Below is a detailed description of an operating IST-PT system followed by specifications and performance data.

Prisoners in Hot Water: Details of an IST Parabolic Trough Installation at the Federal Correctional Institution, Phoenix, Arizona

The parabolic trough solar system at the Federal Correctional Institution (FCI), Phoenix comprises 18,000 square feet of solar collectors. It provides over 70 percent of the facility's annual need for hot water. Because of the solar system, according to Frank Foster prison facilities foreman, “We save a bunch of money on (electric water heater) elements, maintenance calls and repairs. The calls we’ve gotten from the inmates about cold water have basically gone away.” [1] O&M savings are in addition to the savings due to reduced utility charges.

The FCI solar system serves the hot water heating needs of approximately 1100 inmates and staff. IST designed, fabricated and installed the system under an Energy Service Performance Contract (ESPC) with the Federal Bureau of Prisons. This was the first renewable energy ESPC to be contracted with the federal government. The contract term is 20 years, after which time ownership of the solar system reverts to the federal government. IST maintains the plant with the help of a local solar company, North Canyon Solar.

The solar field consists of 120 parabolic trough concentrator modules with a total gross collectorarea of 18,000 square feet (17,000 square feet net area). The modules are arranged in 10 parallel rows. Total system flow is about 100 gpm. Flow is divided into five U-loops in parallel. Using a multi-row drive system, the entire solar field tracks the sun using only four drive/control units.

The solar system operates unattended. Collectors track the sun continually during the day to heat circulating propylene glycol antifreeze solution. Heat from the solar collectors is transferred through immersed copper coils to two hot water storage tanks with a total volume of 23,000 gallons. In the summer, these tanks can heat to the high temperature limit of approximately 185°F (85°C).

Incoming domestic water is heated as it flows through a second set of copper coils immersed at the top of the tanks. Hot water exiting the tank coils is tempered to 135^oF for delivery to the jail through an extensive underground piping system. This piping system serves the needs of five housing units and the central kitchen and laundry. Energy stored in the tanks allows solar-heated hot water to be delivered around the clock. Over the course of a year, over 70% of the hot water needs of the institution is supplied by solar energy. Prior to the installation of the solar system, all water was heated by electricity.

Under solar peak conditions and when the modules are clean, the solar system delivers about 3.0 million Btu/h (850 kW) of heat to the energy storage tank at an efficiency of about 60% of the solar energy incident on the solar collectors. On a sunny day, the solar system delivers 45,000 gallons of hot water to the institution displacing approximately 4,000 kWh of electricity.

The solar system has been running routinely since the beginning of 1999. Each year the solar system delivers about 1.1 million kWh of hot water net of electric power consumption. Peak electric demand at FCI has been reduced by over 200 kW compared to before the solar system was installed.

[1] Mike McCloy, Arizona Republic, June 19, 1999.

Technical Description of IST-PT

IST systems are efficient over a wide temperature range

Thermal Performance

*Dudley, V. E. and Evans, L. R. Test Results: Industrial Solar Technology Parabolic Trough Solar Collector. SAND94-1117, Sandia National Laboratories, Nov. 1995. The curve shown is for a silver reflective film.

Incident Angle Modifier Compared to the Angle Cosine

Concentrator

The IST concentrator is built according to a unique patented design making it very lightweight, yet exceedingly strong. All aluminum construction minimizes concentrator maintenance requirements. The physical characteristics of the concentrator modules are:

Overall Module Size	7 ft. 6 in. x 20 ft.(2.3m x 6.1 m)
Concentrator Weight	178 lb ( 81 kg)
Concentrator Rim Angle	72°
Materials of Construction	Aluminum
Reflective Surface Options	Aluminum acrylic Enhanced polished aluminum

Lightweight,
low maintenance concentrator

Receiver

Reflected solar energy is focused by the parabolic concentrator onto the receiver at the focal point. The receiver comprises a steel absorber, that is coated with a selective blackened nickel surface, and a surrounding envelope of tough Pyrex, glass to reduce heat loss. An anti-reflective coating on the glass increases light transmission. The receiver specifications are:

Absorber Tube Outside Diameter	2.0 inch (5.08 cm)
Absorber Material	Steel
Selective Surface Absorptance Emittance (80°C)	Blackened nickel 0.96 - 0.98 0.15 - 0.25
Absorber Envelope Material	Borosilicate glass
Envelope Anti-Reflective Coating Transmittance	Sol gel 0.95 - 0.965
Maximum Operating Temperature	550°F (288°C)

Light is focused on the receiver tube of the parabolic trough at a concentration of 40 suns

Efficient heat collection

Drive and Controls

The collectors track the sun continuously using the Honeywell Fluxline Control System. A local controller at each drive regulates collector tracking, while a single field controller monitors operation of the overall system. The control system incorporates safety devices that monitor sun, wind, and system temperatures and pressures. A unique multi-row configuration drives up to six rows of troughs in unison (up to a total of 36 modules) so reducing the number of moving parts and increasing reliability. The drive power source is a standard three-phase electric motor.

Honeywell Controls

Flexible Hoses

Fully insulated stainless steel hoses accommodate the motion of the receiver with respect to the fixed field piping.

Long-life hoses

Operating and Maintenance

IST concentrator systems have been operating for 15 years. Basic design concepts have remained unchanged, although the system has been continually improved to increase performance and reliability.

Typically, IST systems run unattended. O&M requirements are minimal. Site inspections every one or two weeks are generally adequate to monitor system operations and to perform routine maintenance. Rain is very effective at washing the reflective surfaces to maintain performance. In dry climates, two months is a typical interval between manual washings of the collectors.

Minimal maintenance and running costs

IST-PT Parabolic Trough Solar Collectors

IST trough systems operate automatically at high efficiency with minimal maintenance. Commercial trough systems can be competitive with natural gas and will deliver energy at a cost not influenced by outside events for more than 20 years.