Current Department of Defense (DoD) energy security strategies for installations include increased use of distributed generation sources, which is currently met by either deploying traditional natural gas- or diesel-fueled generators or installing large-scale renewable energy. However, distributed generation utilizing fossil fuels cannot be used on a regular basis at many DoD installations due to restrictions in air quality permits and interruptions in the supply of natural gas or diesel. Renewable energy resources, notably solar and wind, are inherently intermittent in nature and may not be available when needed without costly grid-scale energy storage. This project will demonstrate the Terrajoule Distributed Generation and Storage (DGS) system—a cost-competitive (less than $0.09/kWh), zero emission alternative to traditional distributed generation systems—at San Luis Obispo, California. The DGS utilizes curved mirrors to collect sunlight for steam production, which is used to make power stored in the compact form of superheated water so that power can be generated later, even when the sun is not shining, providing 24-hour power.

Technology Description

The Terrajoule DGS system is comprised of three major subsystems: the Solar Collectors, Steam Driven Generator Unit, and Superheated Water Energy Storage.

  • Solar Collector Subsystem. Solar concentrators generate steam while the sun is shining. A portion of the received energy is converted immediately to output power and the balance stored for later (night-time) use. For collectors, industry-standard RP-3 parabolic troughs, circulating oil Heat Transfer Fluid (HTF) through heat exchangers to produce superheated steam at 42 bar pressure and 315 degrees C, are currently used.
  • Steam Driven Generator Unit. The steam is converted to mechanical rotating shaft power via a modular steam engine, which is operated at constant, increased, or decreased output depending on load requirements. It is a multistage expansion system with four stages, the first stage consuming steam at up to 42 bar / 315 degrees C and the fourth stage exhausting into a sub-atmospheric condenser at approximately 0.063 bar and 37 degrees C. The design of each engine stage is derived and updated from a Skinner Universal Unaflow design, used for stationary and marine power through World War II. The engine drive shaft is used to drive a standard electric generator.
  • Energy Storage. Energy storage is embedded in the system via superheated water, stored in an insulated pressure vessel at operating temperatures between 130 degrees C and 170 degrees C (corresponding to pressures between 2.7 bar and 7.9 bar). The pressure vessel used is an insulated commercially available 30,000 gallon LPG storage tank. Insulated with 12 inches of standard building insulation, the daily energy loss is less than 2% of stored energy. A single 30,000 gallon storage tank will store sufficient thermal energy that, converted by the engine system to electrical energy, provides 1,200 kWh of electrical energy. Multiple tanks provide energy storage as required. 

This project will demonstrate the ability of the DGS to convert, store, and discharge renewable energy without fossil fuels or large-scale chemical battery energy storage and show that the renewable energy can be used for on-demand power generation as backup to key nodes or provide peak power mitigation.

Implementation Issues

The DGS system can provide on-demand clean distributed power generation on DoD facilities with minimal maintenance and operating cost. It operates as an off-grid, standalone on-demand power source up to 24 hrs per day, and as an economical grid-connected power source that will continue providing power during interruptions of grid power, during sunlight, in cloudy conditions, and after dark. (Anticipated Project Completion - 2015)