Base plant

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).

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