Ashrae Journal - November 2008 - (Page 45)

national emissions requirements and to approach emissions levels acceptable in regions of the U.S. with the most stringent emissions criteria, such as California.1 Finally, the systems could include the capability to provide backup electric power during interruptions in service from the grid. If the system is installed indoors, it requires a reliable, safe, and cost-effective method to reject excess waste heat to the outdoors when providing backup power in situations with limited or no heat demand.3 Despite these advantages, Stirling engine-based RCHP faces several major challenges in realizing appreciable market penetration. The net electrical efficiency of the Stirling engine poses the greatest barrier to widespread use of RCHP for grid-connected applications. To significantly reduce energy costs, the prime Market Factors mover should have electricity generation efficiency similar to Stirling engines have been under development for decades for that of the grid. Neither field-tested systems, nor systems una variety of space, solar, vehicle-propulsion, portable power, and der development, approach these performance levels at RCHP RCHP applications. Today, approximately a dozen organizations scales. Increasing electricity generation efficiency is possible, but worldwide are working to develop KSEs or FPSEs. In addition, would require operating at higher high-end temperatures.1,2,4,5 at least one manufacturer sells a Stirling-based RCHP system Unfortunately, this usually requires using higher performance in Japan, with multiple (and cost) materials manufacturers expectfor the heat input porSteady Heat ing to launch similar tion of the engine, and Input products in Europe also increases emisDisplacer within the next year. sions challenges.1 Piston Regenerator However, both RCHP The limited energy Steady and Stirling engines savings of the current Cooling comprise negligible systems also makes portions of the global their economics less Linear space heating market. favorable. For examAlternator Stator Several strengths of ple, a field study of 1 Stirling engines drive kW (electric) Stirling the interest in develengine-based RCHP oping Stirling enginein the UK estimates Linear based RCHP systems, that the system reElectrical Output Power Alternator Piston including low noise, duces energy costs for Mover fuel flexibility, low a house with an annual emissions, and backup heating demand of power provision. Figure 1: Schematic of a free piston Stirling engine (for electricity generation only).4 22,000 kWh (therMany of the attracmal) by between $70 tive qualities of a Stirling engine derive from the external and $165.† The utility rate structure has a significant impact combustion process it uses. Because its combustion is steady on the savings, i.e., the values (low-end-value) electricity sold instead of intermittent and explosive (as in an internal combus- to the grid at 50% of the retail electric price, while the hightion engine [ICE]), the combustion process can be quiet, e.g., end-value values electricity at the full residential rate (i.e., net similar to a flame on a gas range. Similarly, because Stirling metering). Based on a projected $1,100 cost premium (relative engines simply require a steady heat source, their combustors to a condensing boiler) at larger production volumes, a system can be designed to operate using multiple fuels, such as natural would pay back (simple payback period [SPP]) in approximately gas, propane, heating oil, and diesel fuel, over a wide heat input seven and 16 years for the two rate structures described.2 Another study evaluated the potential energy cost savings range. In practice, the potential for pollutants in the combustion products that can corrode the system limit the range of fuels of Stirling engine-based RCHP for a 3,000 ft2 (280 m2) house located in a Washington, D.C. climate (generally representative used by most systems.7 Furthermore, Stirling engines can achieve low emissions of criteria pollutants relative to ICEs. This enables them to satisfy † Based on a currency conversion rate of $1.84 per British Pound. November 2008 ASHRAE Journal 45 achieve energy savings, the RCHP should only operate when heating demand at least equals the thermal output of the RCHP system, i.e., a thermal load following strategy. One study found that RCHP operation that tracked electricity demand (without net metering) must have an electricity generation efficiency of at least 20% to generate any energy cost savings.3 Using a thermal load following strategy, Stirling-based systems can achieve moderate energy cost savings.2,3 Sufficient thermal loads to operate the system will occur more frequently in residences with higher space heating loads, typically due to some combination of a colder climate, high shell loads (due to poor insulation and/or fenestration), and/or higher floor space. Developers of Stirling RCHP products for the UK have identified larger, older (pre-1920) homes as a promising market.2

Table of Contents Feed for the Digital Edition of Ashrae Journal - November 2008

Ashrae Journal - November 2008 Contents Commentary Industry News Meetings and Shows Feature Articles Reducing Data Center Energy Consumption Building Sciences Feature Articles Sustainable Nature Center Emerging Technologies Special Products Washington Report Special Supplement: BACnet® Today BACnet® Goes Green Seeing the Light with BACnet® BACnet® on Campus BACnet® for Net Zero Analyzing BACnet® BACnet® at Xerox Campus BACnet® in Europe BACnet® for Green Library BACnet® at the University of Minnesota: Flexibility, Interoperability InfoCenter Product Showplace Classified Advertising Advertising Index

Ashrae Journal - November 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.