APC 400kW1MW User Manual download pdf (Page 40)

If boilers are used to supply supplementary heat to the absorption chillers, they will cycle

on and off as the cooling load varies. When operating parameters are met, the boiler operating

controls will shut off fuel flow [51, p. 27.6]. There is a time lag associated with the on-cycle of

boilers, which is due to several necessary steps: a firing interval, a post-purge, an idle period, a

pre-purge, and then return to firing [52, para 3]. The purge cycle is necessary before each on-

cycle because it assures that there is no accumulation of explosive gases in the boiler’s fire box.

Not only is there a time lag during their start-up, but boilers are least efficient at the start of the

on-cycle [53, para 3].

During normal operation of the HVAC system, the fuel cell will always provide waste

heat to the absorption chillers, as it must continually operate in order to provide power to the

facility. However, if the fuel cells are shutdown for maintenance or fail, their waste heat will not

be available to power absorption chillers. If electric chillers are used for supplementary cooling,

then boilers could be used to provide heat to absorption chillers during a fuel cell outage. This

is a viable option because the boilers would be running continuously, avoiding the on-cycle

drawbacks.

4.2.1.1 Absorption Chillers versus Electric Chillers

There are several differences between electric and absorption chillers that data center

owners should be aware of if they install this type of fuel cell CHP system. Besides the

differences in their compression cycles, the pumping, cooling tower size, and cost of the overall

system is effected depending on the chiller type used.

Cooling towers in absorption chiller systems require approximately 3.6 gallons per

minute (gpm) of condenser water per ton. The typical difference in condenser water entering

and leaving temperatures ranges from 15°F to 20°F with an outdoor wet bulb temperature of

78°F. With electric chillers, 3 gpm of condenser water per ton is common with a 10°F

temperature change at a 78°F wet bulb temperature. Therefore, a larger cooling tower capacity

and more pumping power are required for absorption chillers in comparison to standard electric

chillers [54; 50, p.1; 1, p. 80].

Electric chillers use motor-driven compressors that require a significant amount of

power. In fact, electric chillers typically consume the largest percentage of data center power

and account for considerable portions of annual energy budgets [46, para 2]. Absorption

chillers use less electricity, about 0.02 kW per ton, compared to 0.47 kW up to 0.88 kW per ton

for an electric chiller, depending on the type of compressor. However, compared to electric

chillers, absorption chillers have higher initial costs and are not as widely available. Between the

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APC 400kW1MW User Manual Page 40