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Using stirling engines for residential CHP.

Most households obtain electric power from the grid, space heating from a furnace or boiler, and hot water from a gas-fired or electric-resistance water heater. In contrast, residential combined heat and power systems (RCHP) use a prime mover to generate electric power and harness waste thermal energy produced in the power-generation process to provide heat to satisfy space heating, water heating, and, potentially, space cooling loads (e.g., via absorption cooling). In practice, not all the waste heat is recoverable due to thermal energy radiated to the ambient air, losses from the exhaust-gas stream, low-quality heat (e.g., temperature too low to satisfy household thermal loads), and heat generation that exceeds household thermal loads at that point in time.

Stirling engines are a type of prime mover under development for deployment in RCHP applications. They use an external combustion process to provide heat to a sealed pressure vessel at temperatures of around 500[degrees]C-600[degrees]C (932[degrees]F-1,112[degrees]F) at RCHP scale. (1,2) Inside the pressure vessel, a displacer piston moves a working fluid (typically helium or hydrogen) between the hot and cold sides of the engine (Figure 1). High engine efficiency depends heavily on the operation of the regenerator, a thermal capacitor that stores and returns heat to the shuttling gas as it moves between the hot end and the cold end of the engine, thereby decreasing the amount of fuel needed to raise the gas temperature on the hot side, and decreasing the cooling requirements on engine cold side. As the gas is heated and cooled, the gas pressure inside the vessel oscillates about its mean value. Consequently, the gas has the potential to do work on a "power piston," driving it back and forth against an opposing load, e.g., an electrical alternator.

There are two main categories of Stirling engines, kinematic and free-piston. Kinematic Stirling engines (KSE) employ mechanical linkages to coordinate displacer and power piston motions and convey linear piston power to a rotary alternator. Free piston Stirling engines (FPSE) do not have mechanical linkages but, instead, employ a balance of pressure, spring and alternator load reaction forces to achieve the same functionality.

KSE and FPSE both have advantages. In one regard, KSE control is simpler since displacer and power piston motions are mechanically coordinated. The FPSE must have the means to effectively control piston excursions to avoid collisions with the pressure vessel, displacer piston, or additional power pistons in the engine. On the other hand, this also enables continuous modulation of piston stroke, offering an opportunity to optimize efficiency at partial load. An important feature for FPSE is that the elimination of mechanical linkages can enable design for maintenance-free operation for tens of thousands of hours.

When used for RCHP, a liquid cooling loop recovers reject heat from the cold end of the Stirling engine and its combustion exhaust gases. This waste heat is then transferred to a storage tank or circulated through a hydronic system to provide space and water heating. In addition, some RCHP systems nearing commercialization include an additional burner to augment the space heating capacity of the unit so it can meet all space heating loads, obviating the need for an additional space heating source. (2)

Energy Savings Potential

The electric generation efficiency of Stirling engines integrated with RCHP systems are appreciably less than that of the electric grid. For example, although RCHP-scale Stirling engines can achieve electric generation efficiencies on the order of 10% to 20% * (low heating value [LHV]), (1,3,4) field tests of preproduction RCHP systems indicate typical net system electrical efficiencies (i.e., taking into account pump and fan parasitics, the impact of transient operation) of between 6% and 8%. (2,5) For comparison, the U.S. electric grid has an average efficiency of approximately 30% (high heating value [HHV], taking into account transmission and distribution losses). (6) On the other hand, the liquid cooling loop can recover approximately 80% of waste heat, (3) yielding an overall thermal efficiency of about 70% in field tests. (2)

Consequently, for Stirling engine-based RCHP systems to reduce primary energy consumption, they must not just recover but also use a large portion of the waste heat to supplant space and hot water heating loads while generating a relatively small quantity of electricity (typically on the order of 1 kW). In turn, this underscores the importance of the RCHP operating strategy in determining when to run the Stirling engine. Specifically, to 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)

Market Factors

Stirling engines have been under development for decades for a variety of space, solar, vehicle-propulsion, portable power, and RCHP applications. Today, approximately a dozen organizations worldwide are working to develop KSEs or FPSEs. In addition, at least one manufacturer sells a Stirling-based RCHP system in Japan, with multiple manufacturers expecting to launch similar products in Europe within the next year. However, both RCHP and Stirling engines comprise negligible portions of the global space heating market.

[FIGURE 1 OMITTED]

Several strengths of Stirling engines drive the interest in developing Stirling enginebased RCHP systems, including low noise, fuel flexibility, low emissions, and backup power provision.

Many of the attractive qualities of a Stirling engine derive from the external combustion process it uses. Because its combustion is steady instead of intermittent and explosive (as in an internal combustion engine [ICE]), the combustion process can be quiet, e.g., similar to a flame on a gas range. Similarly, because Stirling engines simply require a steady heat source, their combustors can be designed to operate using multiple fuels, such as natural gas, propane, heating oil, and diesel fuel, over a wide heat input range. In practice, the potential for pollutants in the combustion products that can corrode the system limit the range of fuels used by most systems. (7)

Furthermore, Stirling engines can achieve low emissions of criteria pollutants relative to ICEs. This enables them to satisfy 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 mover should have electricity generation efficiency similar to that of the grid. Neither field-tested systems, nor systems under development, approach these performance levels at RCHP scales. Increasing electricity generation efficiency is possible, but would require operating at higher high-end temperatures. (1,2,4,5) Unfortunately, this usually requires using higher performance (and cost) materials for the heat input portion of the engine, and also increases emissions challenges. (1)

The limited energy savings of the current systems also makes their economics less favorable. For example, a field study of 1 kW (electric) Stirling engine-based RCHP in the UK estimates that the system reduces energy costs for a house with an annual heating demand of 22,000 kWh (thermal) by between $70 and $165. ([dagger]) The utility rate structure has a significant impact on the savings, i.e., the values (low-end-value) electricity sold to the grid at 50% of the retail electric price, while the high-end-value values electricity at the full residential rate (i.e., net metering). Based on a projected $1,100 cost premium (relative to a condensing boiler) at larger production volumes, a system would pay back (simple payback period [SPP]) in approximately seven and 16 years for the two rate structures described. (2)

Another study evaluated the potential energy cost savings of Stirling engine-based RCHP for a 3,000 [ft.sup.2] (280 [m.sup.2]) house located in a Washington, D.C. climate (generally representative of entire country), using higher utility rates. ([double dagger]) Assuming a 1 kW (electric) Stirling engine operating at 15% (LHV) electricity generation efficiency and using a thermal and electric load following strategy, ([section]) the system would reduce annual energy costs by about 12%. With net metering, the energy cost savings increased to about 17%. This study also estimated an installed system cost (when produced at significant volume) of around $7,000, with two important changes: it does not serve service water heating loads and provides back-up power via lead-acid batteries. (4) Assuming that the backup power capability adds $1,000 to the cost, the RCHP system has a cost premium of around $2,250 relative to an ENERgy STAR[R] boiler. (8) Based on the utility cost structure noted earlier, it would realize annual cost savings of around $280, (4) yielding a payback of about eight years.

Of course, energy cost savings will decrease--and SPP will increase--as heating demand decreases, e.g., for more efficient homes. (2,3)

Both analyses reinforce how greater availability of net metering would improve the economics of RCHP; in essence, net metering allows the system to always follow the thermal load because any excess electricity will always be used. Although the Energy Policy Act of 2005 requires public utility commissions to consider net metering, it is not clear that this legislation will increase net metering availability for nonrenewable generation technologies. (4)

Achieving the long lifetimes characteristic of furnaces and boilers, e.g., on the order of 20,000 to 40,000 hours, is another challenge for Stirling engine-based RCHP. Because the engines are sealed, the internals of Stirling engines cannot be lubricated, which makes achieving such long lifetimes very challenging. Although both KSE and FPSE configurations have demonstrated operation in excess of 5,000 hours, (1) it is not clear is that they have demonstrated the longevity required for RCHP.

Finally, European and Japanese RCHP products typically use IC engines, Stirling engines, or fuel cells, but these products are not compatible with the forced hot-air heating systems that dominate the U.S. housing stock. Furthermore, these products cannot provide backup power. (3)

References

(1.) EPRI. 2005. "Technology Review and Assessment of Distributed Energy Resources." Final Report prepared by TIAX LLC for the Electric Power Research Institute Inc. No. 053828. http://tinyurl.com/epri2005 (or http://mydocs.epri.com/docs/public/000000000001012983.pdf).

(2.) Carbon Trust. 2007. "Micro-CHP Accelerator--Interim Report." The Carbon Trust. http://tinyurl.com/carbontrust2007 (or www.carbontr ust.co.uk/publications/publicationdetail.htm?productid=CTC727).

(3.) Zogg, R.A., et al. 2005. "Micro-generation appliances for the U.S. Market." Proceedings of the 56th International Appliance Technical Conference and Exhibition.

(4.) TIAX. 2006. "Research, Development, and Demonstration of Micro-CHP Systems for Residential Applications--Phase I." Final Report by TIAX LLC to the U.S. Department of Energy, Office of Electric Delivery and Energy Reliability, Report No. DOE/ NT/4221401. http://tinyurl.com/tiax2006 (or www.propanecouncil.org/ uploadedFiles/11538_TIAX_MicroCHP_Report_FINAL.pdf).

Bell, M., et al. 2004. "Testing residential combined heat and power 5. systems at the Canadian centre for housing technology." Proceedings of the ACEEE Summer Study for Energy Efficiency in Buildings. http://tinyurl.com/bell2004 (or www.eceee.org/conference_proceedings/ ACEEE_buildings/2004/Panel_11/p11_1/).

(6.) DOE. 2008. Buildings Energy Data Book. U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy. http://buildingsdatabook.eren.doe.gov/.

(7.) Goldstein, L., et al. 2003. "gas-Fired Distributed Energy Resource Technology Characterizations." National Renewable Energy Laboratory Final Report. NREL/TP-620-34783. http://tinyurl.com/goldstein2003 (or www.nrel.gov/docs/fy04osti/34783.pdf).

(8.) Navigant Consulting. 2007. "EIA Technology Forecast Updates--Residential and Commercial Building Technologies--Reference Case Second Edition (Revised)." Presented to the U.S. Department of Energy, Energy Information Administration.

* Larger Stirling engines (tens of kW) can achieve peak efficiencies of approximately 30%. (1)

([dagger]) Based on a currency conversion rate of $1.84 per British Pound.

([double dagger]) Those typical of New York State circa 2003, i.e., $11.44/MMBtu and $0.1431/kWh.

([section]) The RCHP system output is modulated so that both its entire electric and thermal outputs can be used by the household. That is, sometimes electric loads will limit CHP system operation and thermal loads will limit operation at others. (4)

By Kurt Roth, Ph.D., Associate Member ASHRAE; Jason Targoff; and James Brodrick, Ph.D., Member ASHRAE

Kurt Roth, Ph.D., is a principal, and Jason Targoff is an associate principal with TIAX LLC, Cambridge, Mass. James Brodrick, Ph.D., is a project manager with the Building Technologies Program, U.S. Department of Energy, Washington, D.C.
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Title Annotation:Emerging Technologies; combined heat and power
Author:Roth, Kurt; Targoff, Jason; Brodrick, James
Publication:ASHRAE Journal
Geographic Code:1USA
Date:Nov 1, 2008
Words:2247
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