Over the next two decades, the Department of Defense (DoD) intends to dramatically increase its usage of renewable energy. This is part of a concerted effort to reduce life-cycle costs and greenhouse gas (GHG) emissions. The objectives of this project were to demonstrate an innovative hybrid electric/thermal solar cogeneration system at Port Hueneme (Naval Base Ventura County) and the Parks Reserve Forces Training Area (PRFTA) (Dublin, CA); validate and document performance and cost advantages; and develop financing models and engineering tools to expedite transfer of the technology widely across DoD facilities.

Technology Description

Cogenra Solar’s SunDeck system combines proven photovoltaic (PV) and solar hot water (SHW) technologies into a single integrated solar cogeneration system that extracts as much of the sun’s incident power as possible, as high-value electricity and delivers the rest as useful heat. The solar collectors are water-cooled concentrating PV parabolic troughs that capture rather than dissipate what other PV approaches call “waste heat.” The architecture comprises a series of ground or roof-mounted arrays that independently track the sun along one axis. Within each array, a series of flat mirrors concentrate sunlight (~8X) onto silicon-based PV-Thermal panels that generate electricity. Conduits in the receiver panel carry a water-glycol mixture in a closed loop that cools the PV cells, enhancing their performance and capturing the excess solar energy as heat. A compact SHW heat exchange/storage system transfers the heat to preheat the domestic water supply before it enters the site’s pre-existing hot water heater.

Demonstration Results

This demonstration project included the installation of Cogenra systems at five separate buildings; three at Port Hueneme and two at PRFTA. The electricity and thermal energy delivered by these systems was measured for 1 year, and the systems continue to operate.

The project set out to demonstrate that compared to standard PV and SHW arrays of the same size, Cogenra’s system achieves the following:

  1. Generates at least 4.75X as much renewable energy (electricity + heat).
  2. Delivers 2X the economic value.
  3. Reduces GHG emissions by 2.6X versus PV and by 1.3X versus SHW.
  4. Pays back the initial investment in energy cost savings in less time.
  5. Accelerates compliance with DoD energy and environmental goals ~2X.
  6. Requires minimal operation and maintenance, comparable to PV and SHW.

The SunDeck systems performed well and delivered over 4X the renewable energy as a reference PV array, 1.7X the economic value as a reference PV array, and 1.4X the value of a reference SHW array. These gains were somewhat less than the stated performance goals, primarily due to inconsistent hot water usage in some of the buildings, especially the barracks. Similarly, the SunDeck systems demonstrated greater GHG emissions reduction than PV or SHW, though slightly less than the target amount due to system utilization. Low or inconsistent hot water demand limits the utilization of the cogeneration system overall, but especially the amount and value of the heat delivered. When there is low demand for the thermal energy collected by the solar array, the solar thermal storage tank heats up and eventually reaches its upper temperature limit, which triggers the array to de-track to avoid overheating. When de-tracked, the array produces neither electricity nor hot water, impacting the economics of the project. In cases where hot water demand is inadequate, Cogenra’s system architecture can be configured to cheaply dissipate some or all of the captured heat.

Implementation Issues

This project demonstrated the ability of Cogenra Solar’s SunDeck system to generate significantly more renewable energy, energy value, and GHG reductions compared to the widely available PV and SHW technologies, while also reducing cost. Lifecycle cost analysis demonstrated that the Cogenra systems offer a payback period of 5.1 years, ½ to ⅔ the payback time of PV or SHW. Overall, the project demonstrated the increased value of cogeneration, enabling accelerated and cost-effective compliance with DoD’s energy and environmental goals. Operation and maintenance requirements have been similar to PV or SHW, and the systems continue to operate successfully.