Dish Stirling.pdf

Shown above is the free-piston Stirling engine, linear alternator, and

heat-pipe receiver used in the Cummins 7-Kwe system. Solar energy

collected inside the hemispherical absorber of the heat-pipe receiver

(bottom)is transferred to the horizontally opposed - and balanced -

Stirling engines. Mechanical output from the power pistons is converted

to alternating current electricityby the linear alternators.

A Cummins Power Generation

dish-Stirling system in Abilene,

Texas (left),converts the sun’s

energy to electricity to pump

water in mal locations.

The system uses sodium to

transfer heat from sunlight to the

engine; the sunlight is collected

by an array of stretched-

membrane mirrors.

DISCLAIMER

Portions of this document may be illegible

electronic image products. Images are

produced from the best available original

document.

ID I1 s IHl5T

TheDepartment of Energy through its national laboratorieshas

been developing solar systems to help provide clean energy for the

nation. The systems range in size from huge central receiver power

plants like Solar Two, capable of producing megawatts of electricity,

to photovoltaic arrays that simply power one vaccine refrigerator or

water pump.

]Beingperfected now as a joint venture between Cummins Engine

Company and Sandia National Laboratories is a technology that can

supply power for such different users as utility companies and remote

villages. The dish-Stirling solar electricsystem they are developing uses

the versatility of a Stirling engine, which is powered by a heat source,

and a dish-shaped solar concentratorto provide heat from the sun.

supported by the DOE since the 1970’s, dish-Stirling technology

has reached peak net efficiencies of more than 29 percent, meaning it

can convert more than 29 percent of the sunlight that illuminates the

collector into electricity, a world record. Developmentcosts of the joint-

venture program are being shared equally by Cummins, including its

industrial partners, and the Department of Energy through a 5-year,

$17-miUion program. Three generations of Cummins dish-Stirling sys-

tems will be fielded and tested at sites across the United States.

For the past few years Cummins, a leading manufacturer of

diesel engines and generators, has been working on dish-Stirling solar

thermal technology to provide existing diesel generator customers

with a new product and to expand Cummins’ existing customer base.

with the commercialization of dish-Stirling systems and

increased demand, the price to build them is expected to drop, and

consequently, so will the price of electricity they produce. The lower

price, in turn, promises to encourage use of dish-Stirling systems, par-

ticularly on remote areas.

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THE SYSTEMS ARE SIIMIPILE A D IFUELED BY iHlE SUI

Dish-Sterling technology uses a sun-tracking system to con-

centrate solar energy onto a free-piston Stirling engine that has only

two moving parts and requires no oil lubrication. The moving parts

are pistons - a displacementpiston and power piston.

lrhe engine has no crankshafts, no cams, no connecting rods. Its

design is different from that of a kinematic Stirling engine, which

like most engines has linkages and other moving parts, all of which

have to be oiled.

power the engine, an array of curved mirrors made of

stretched reflective membranes collects sunlight and focuses it onto

a receiver. The intense heat vaporizes liquid sodium inside the

receiver, which xondenses on heater tubes and heats helium gas

inside the engine. The expanding helium drives the pistons, which

in turn drive an alternator to produce electricity.

THE ~~~~E~~ ARE AILS0 MODULAR

lrhe system produces nominal 240-volt electricity, and one unit

can generate more than 7 kilowatts of power, enough to provide

power to a remote village or hotel site or to pump water. Individual

units can be used by themselves or they can be connected to a utility

grid. Like other kinds of solar energy, dish-Stirling solar thermal sys-

tems do not produce pollution.

A plus for the system is that the engine can be heated with an

auxiliary fuel such as natural gas, propane, or heating oil when the

sun doesn’t shine. The system has a microcomputer that calculates

the position of the sun and keeps it focused on the sun.

hectricity can cost from 30 to 60 cents per kilowatt-hour in

remote markets - compared with 8 to 10 cents for the same amount

of power in the southwestern United States. Cummins plans to

address the high-value remote power markets first with the 7 kilo-

watt system. Sandia and Cummins expect that when the technology

is technically mature, dish-Stirling systems will be able to compete in

utility markets as well.

FOR M0kE INFOWvlATION

For more information about dish-Stirling systems or the joint

venture, call Rich Diver at Sandia National Laboratories, Solar

Thermal Technology Department, 505-844-0195.

SAND93-2734

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