Whether to attain a higher green-building rating, create a more progressive corporate image, attract environmentally conscious tenants, and/or keep in step with private and public initiatives, many building owners and mechanical-system designers are looking to reduce greenhouse-gas emissions, chiefly carbon dioxide (CO₂) associated with the consumption of fossil fuels.

This article discusses how the selection of one of a building's major energy users — central-plant HVAC equipment — impacts not only utility bills, but greenhouse-gas emissions, and examines several technologies that make reduced emissions possible, often while saving enough energy to pay for themselves.

SCOPE OF THE PROBLEM

According to the 2007 Buildings Energy Data Book,1 commercial buildings consume 35 percent of the electricity used in the United States. Most of that electricity is generated by coal-fired power plants, which discharge CO₂. In 2005 alone, the U.S. buildings sector was responsible for CO₂ emissions of about 250 million tons.

For every 1.00 kwh of electricity produced using the U.S. national mix of fuel sources, a power plant emits 1.30 lb of CO₂.² Compounding the problem, most of the plant's energy output is wasted. Sixty-four percent of the source energy consumed is lost as heat, while 9 percent is lost in transmission. So, by the time electricity reaches a building's HVAC equipment, only about 27 percent of the energy remains to do useful work (Figure 1).

For example, consider a medium-sized hospital with peak loads of 2,500 tons (8,800 kw) of electric cooling, 15 million Btuh (4,400 kw) of natural-gas heating, and 5 MW of power. The building's load profile is shown in Figure 2, which is based on a “typical” 24-hr day during each of the four seasons. Note that while the cooling load varies greatly throughout the year, the heating and electric (less cooling) loads are relatively constant.

Simple analysis shows the hospital consumes 30.4 million kwh of electricity and 25.5 million kwh of natural gas per year (Table 1). As a result, it emits about 50 million lb (25,000 tons) of CO₂ annually. With electricity costing 12 cents per kilowatt-hour and gas costing 3.6 cents per kilowatt-hour (equivalent to $1 per therm), the hospital's annual utility bill is about $4.6 million.

COMPARISON

CO₂ generated by electric HVAC equipment can be reduced two ways:

• By reducing the electricity consumption of the equipment.

• • By utilizing an energy source other than electricity.

The remainder of this article is dedicated to an evaluation of central-plant HVAC equipment that may be able to cut CO₂ emissions using one of these two approaches. Specifically, the chillers, boiler, and electric power of the example medium-sized hospital will be compared with three alternative technologies: gas-engine chillers with exhaust-heat reclaim, water-to-water heat pumps, and combined heat and power (CHP). The objective is to determine if these technologies can accomplish a significant greenhouse-gas reduction (GGR) and offer a return on investment (ROI).

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