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.
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).
The hospital's central plant consists of three 833-TR centrifugal chillers, one 5-MW boiler, and a 5-MW utility power supply (Table 2). Because this is the base plant, there is no payback, ROI, or GGR.
In the comparison of each of the three alternative HVAC technologies with this base plant, the quantities of cooling, heating, and power remain constant.
Obviously, the economic and environmental impact of each alternative technology depends on the application. Each building has a unique load profile, and energy costs vary. Thus, the numbers produced by this comparison indicate a particular outcome and should be treated as a starting point for an analysis of a specific building.
Gas-engine chillers with exhaust-heat reclaim
With gas-engine chillers with exhaust-heat reclaim, energy loss would be about 9 percent, meaning 91 percent of the source energy would be available for use in the building (Figure 3).
This solution would reduce the electricity requirement of the hospital by more than 12 percent, from 30.4 million kwh to 26.6 million kwh. It would reduce the CO2 generated by the use of utility electricity by the same percentage, from 19,700 tons to 17,300 tons.
As chiller loading varied, heat — ranging from 4 to 80 percent of the design heating load and averaging about 40 percent annually — would be produced. With the hospital having a year-round cooling load, this heat could be used to offset some of the heat required from the boiler. As a result, the energy consumption and operating cost of the boiler would be reduced by 40 percent.
Overall, gas-engine chillers with exhaust-heat reclaim would reduce energy costs from $4.6 million to $4.1 million, a savings of nearly 11 percent. The amount of CO2 generated would be reduced from 25,000 tons to 22,600 tons, a savings of nearly 10 percent. The energy savings would be enough to pay for the higher capital and maintenance costs of the chillers in about two years, for an ROI of about 50 percent (Table 3).
Water-to-water heat pumps
Simply stated, water-to-water heat pumps are repurposed electric-drive chillers. They recycle low-grade heat, reducing the consumption of gas for heating. The heat source is the year-round cooling requirement of the building core, so both useful cooling and heating are produced. Figure 4 is a simple illustration of one possible piping design.
With coefficients of performance as much as four times higher, electric heat pumps produce hot water much more cost-effectively than fossil-fuel boilers. Even as electricity costs go up, the reduction in gas costs typically reduces overall energy costs.3 Overall CO2 emissions typically are reduced as well.
In our example hospital, two 600-TR electric heat pumps would be coupled with two 600-TR electric chillers. With the cooling load exceeding the heating load during most hours of operation, the heat pumps could carry most of the heating requirement, with the boiler operating in a supplementary role. This would result in a nearly 89-percent reduction in boiler operating hours (8,760 hr to 964 hr), energy use (25.5 million kwh to 2.9 million kwh), and CO2 emissions (5,400 tons to 600 tons). Additionally, the size of the boiler would be reduced by 60 percent. Electric service, however, would have to be increased by 10 percent to handle the additional load of the heat pumps.
Although there would be a rise in energy costs and CO2 emissions associated with the electricity used for the chillers and heat pumps, there would be an approximately 11-percent reduction in the central plant's combined gas and electricity costs and CO2 emissions.
The energy savings, plus the reduced capital costs of the chillers and boiler, would be enough to offset the added capital costs of the heat pumps in about one year, resulting in an ROI of about 100 percent (Table 4).
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).
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.
DOE. (2007). 2007 Buildings energy data book. Washington, DC: U.S. Department of Energy. Available at http://buildingsdatabook.eere.energy.gov/
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
Temos, E. (2006). Using waste heat for energy savings. ASHRAE Journal, 48, 28-35.
For past HPAC Engineering feature articles, visit www.hpac.com.
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.