Parabolic Trough Solar Concentrator Technology

by K Sheinkopf, CADDET US National Team and K May, Industrial Solar Technology, USA

INTRODUCTION

For the past seven years, a parabolic trough concentrating collector system has reliably and efficiently provided high-temperature pressurised water for a state prison housing 5,100 inmates in southern California.

The system, located at Tehachapi, has demonstrated that solar energy systems can be cost-competitive with higher-cost fuels, and it will even achieve cost-competitiveness with natural gas if current price trends continue.

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                View of the parabolic collector system in Tehachapi, California

THE SYSTEM

This solar system was designed, manufactured, installed and is operated for the system owners by Industrial Solar Technology, a solar engineering company located in Golden, Colorado.

The system is simple, reliable and efficient. When the sun shines, a glycol/water anti-freeze mixture is circulated through the 192 parabolic trough solar collectors in a closed-loop, to a heat exchanger located in the boiler room about 130 m from the solar field. Here, energy is transferred to a pressurised, high-temperature water loop at 104 oC. The heated water is then circulated throughout the institution to supply domestic hot water for the showers, laundry, kitchen and for space heating. There is no thermal storage since the load absorbs the entire output of the 2,677 m2 solar field.

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Schematic of the parabolic trough collector system

The solar collectors, which are arranged in 16 rows configured in eight parallel flow paths, are co-ordinated through a single master controller which tracks the sun continuously during the day.

The key components of the system include:

  • 6.1 m long troughs with a geometric concentration ratio of 45 based on an aperture width of 2,286 mm and an absorber tube of 51 mm outside diameter;
  • reflective film made of an aluminised acrylic film, laminated to the aluminium concentrator structure which weighs about 5.5 kg/m2;
  • an absorber tube coated with a black chrome selective surface and is surrounded by borosilicate glass tubing;
  • a multi-row drive system whereby 335 m2 of collector area tracks the sun using a single drive/controller mechanism

These design features lowered the cost of the system so that the energy it provides is competitive with conventional fuels. The collector outlet temperature is 147 oC at design conditions. At maximum output, the system has a conversion efficiency (direct solar radiation to delivered thermal energy) of 46%, and delivers 1.2 MWth.

Electrical input, mainly to drive the circulating pump, is less than 5 kW. During the summer months when the sun is shining, the system provides nearly all the process heating needs of the prison.

Major system components are expected to last for 30 years, with replacement of the reflective film and the flexible hoses planned after 15 years. Local personnel, who perform the system maintenance, report that their most time-consuming task is washing the reflective system every two months.

One interesting aspect of this project is that the strong winds in the Tehachapi area have made it an ideal location for wind turbines. The prison solar system thus had to be designed to cope with this environment, allowing it to withstand winds in excess of 130 km/h.

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Side view of a parabolic module

ECONOMICS

This retrofit, turnkey solar system was sold to the private third-party owners for $630,000 ($235/m2), where $ is the US dollar. The sale of thermal energy to the prison is repaying this capital investment. Solar energy to the prison is priced at 5% less than the cost of burning natural gas in the existing boilers to produce an equivalent amount of energy. It is expected that the system will save more than 7 billion Btu per year of natural gas (2 GWh or in fuel oil terms about 200,000 litres) that would otherwise have been burned to meet the energy needs of the prison.

SUMMARY

About 5 x 1015 Btu of energy are used each year in the United States to deliver energy in the form of hot water, hot air or steam at temperatures less than 250 oC. Parabolic trough solar collectors have proven to be the most advanced solar technology that can meet the demands of this market. Systems installed at commercial facilities around the country are proving the economic and technical value of parabolic trough concentrators for more dispersed thermal and solar absorption cooling applications.

The optical efficiency of the collector in this system is 65%. Newer systems have incorporated improved optical materials, including a silver acrylic reflective surface, anti-reflective coatings on the glass absorber annulus and a blackened nickel selective surface a combination of improvements that has increased the optical efficiency to above 76%.

Parabolic trough systems are thus a reliable alternative to the burning of fossil fuels for a wide variety of applications. They reduce pollution and provide long-term insurance against price increases of conventional fuels.

For more information contact the CADDET US National Team in Golden, Colorado.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to
Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.