Advances in Solar Thermal Electric Power Generation

by R Franklin, CADDET US National Team

Introduction

As an alternative to photovoltaic systems for electric power applications in village and other remote power markets, Cummins Power Generation (CPG) is fine tuning a 7-kWe solar thermal device that marries concentrated heat energy from the sun with a unique helium-based engine.

The Dish-Stirling System

The United States Department of Energy (DOE) has supported solar thermal concentrator technology since the 1970s, through programmes that develop and test systems for both low-power remote electric generation and larger grid-connected generation. One of these programmes is the Dish-Stirling Joint Venture Programme (DSJVP), a three-phase project begun in 1991, and now into its third phase. The objective of DSJVP is to commercialise a 7-kWe solar thermal electric system by the year 2000.
 In developing its CPG-460 7-kW
e concentrator system, the company received technical assistance from Sandia National Laboratory on behalf of DOE. At present, ten of these solar concentrators are employed in the field. Four are under test at the company's test site in Abilene, Texas. Additional systems are being evaluated for off-site testing in Texas, Pennsylvania, California, France and Japan.
 The 7-kW
e concentrator system can provide clean, low-cost energy for applications such as rural water pumping or remote village electrification. The system is unique in that it uses an externally-fired heat engine. This permits the use of any heat source that is capable of providing sufficient temperature.
 The system is environmentally benign and releases no emissions from its engine. Instead of ingesting fresh, cool air and exhausting the residuals of hot expansion, the unit employs a fixed mass of fully-enclosed gas over and over again. The ground space required for placement and construction of the concentrator foundation is minimal and presents no significant impact to the surrounding area.
 One hybrid concentrator unit has been developed that permits use of two fuel sources: sunlight and natural gas. This fuel flexibility helps overcome the intermittent nature of the solar resource and allows users to keep the dish-Stirling engine on line at all times, maximising the potential of their capital investment.

System Components

The CPG-460 7-kWe dish-Stirling system includes:

  • a solar concentrator;
  • a heat pipe receiver;
  • a free-piston Stirling engine with a linear alternator;
  • system controls, protection and power conditioning;
  • a cooling sub-system.

The concentrator system utilises 24 mirror facets, each measuring about 60 inches in diameter. A microcomputer calculates the position of the sun and keeps the system focused in its direction. In normal operation the concentrator tracks the sun in two axes and focuses sunlight through the aperture onto the hemispherical absorber of the engine's heat pipe receiver.
 The concentrator panels produce about 30 kW of reflective power to the heat-pipe and engine assembly. The heat-pipe receiver can transfer heat at temperatures ranging from 300-800 degrees C.
 The dish-Stirling engine has no crankshafts, cams or connecting rods. Focused thermal energy from the concentrating mirrors bathes the engine in heat and vaporises liquid sodium in its receiver. Condensation of the sodium on heater tubes raises the temperature in an internal helium gas circuit. The expanding helium drives pistons which, in turn, drive an alternator to produce single-phase, 60 Hz nominal ac power.
 Conversion efficiencies have varied according to the configuration of the engine-alternator assembly. A previous engine-alternator delivered a solar thermal-to-electric conversion efficiency of up to 30%, but encountered difficulty with the helium flow distribution in the engine heater head. A more recent design delivered a 22% conversion efficiency with the use of interim regenerators. As of late 1995 this unit had demonstrated a maximum output of 5.2 kW
e.
 The critical path of development for the 7 kW
e system is a compatible engine-alternator design. CPG's current design is considered attractive because it is well suited to manufacturing. Engineering refinements are being implemented to improve displacer and piston seals and the regenerator assembly.
 Special features being developed for the CPG-460 system are automatic start up and the hybrid option.

Economics

At a production volume of 10,000 concentrator units per year, the total installed cost is estimated at $22,500, or $3,000 per installed kilowatt, at 1994 prices (where $ is the US dollar). This cost compares favourably with other renewable energy technologies, such as photovoltaics and wind turbines.

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.