Combined heat and power

When chilled-water production is integrated into a CHP solution, heat from the electric generator can be used for either heating or cooling. As a result, more of the potential energy in the fossil fuel can be utilized. Of course, this potential can be realized fully only if the facility has concurrent heating and cooling loads year-round.

For medium-size to large commercial and institutional facilities, the components of a CHP system vary, depending on the available power-production capacity. Thus, equipment-room space may be an issue.

With medium-size systems, a natural-gas engine often is used to drive the generator. Engine-coolant and engine-exhaust heat are carried to a heat exchanger, where they can be used to heat the facility. The heat also can be employed to drive a hot-water absorption chiller to supply chilled water to the facility. Packaged systems, which minimize installed cost, combine the absorption chiller, piping, heat exchangers, and control system.

With larger systems, a natural-gas combustion turbine can be used to drive the generator. Heat from turbine exhaust is captured by a heat-recovery steam generator. The steam can be used to heat the facility; it also can be used to drive a steam-turbine chiller to cool the facility.

For our example hospital, a gas-engine system will be evaluated. Based on an analysis of power, heating, and cooling requirements, two 1.2-MW generators are selected. Throughout the year, electricity will continue to be purchased from the utility. One 600-TR absorption chiller and two 900-TR electric-drive chillers also are selected. The 1.0-MW boiler is 80-percent smaller than the capacity required for the base case.

Because gas in the system is consumed for electricity generation as well as heating, gas-energy costs go up about 178 percent ($900,000 to $2.5 million). But because more of the potential energy is utilized, gas-energy CO₂ emissions go down about 15 percent (5,400 tons to 4,600 tons). As expected, electricity-energy costs fall nearly 80 percent ($3.7 million to $800,000), as do CO₂ emissions (19,700 tons to 4,000 tons). Overall, combined energy costs are reduced by about 30 percent ($4.6 million to $3.2 million), while CO₂ emissions are reduced by about 68 percent (25,000 tons to 8,100 tons).

In this scenario, energy savings and reduced boiler cost are enough to pay for the added capital cost of the generators and absorption chiller, plus the added maintenance cost of the generators, in about a year-and-a-half, which results in an ROI of about 75 percent (Table 5).

CONCLUSION

By carefully considering available HVAC and power-equipment choices, building owners and designers can address concerns over the cost of energy and the environmental consequences of using fossil fuels. Table 6 summarizes pertinent data for all of the technologies discussed in this article.

Thanks to these technological options, there is no need to worry about a trade-off between being financially justifiable and environmentally responsible when investing in energy efficiency. That is because equipment choices that address both building efficiency and greenhouse-gas reduction are available. As a result, building owners and designers can make a decision that addresses both economic and environmental challenges.

REFERENCES

1. DOE. (2007). 2007 Buildings energy data book. Washington, DC: U.S. Department of Energy. Available at http://buildingsdatabook.eere.energy.gov/

2. EIA. (2008). Greenhouse gases, climate change, and energy. Washington, DC: Energy Information Administration. Available at http://www.eia.doe.gov/bookshelf/brochures/greenhouse/Chapter1.htm

3. Temos, E. (2006). Using waste heat for energy savings. ASHRAE Journal, 48, 28-35.

For past HPAC Engineering feature articles, visit www.hpac.com.

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At the time this article was written, Ian Spanswick was senior program manager for industrial systems for Johnson Controls. He holds a bachelor's degree in chemical engineering from Loughborough University.

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