APC 400kW1MW User Manual download pdf (Page 42)

4.2.2 Domestic Water Heating in a Fuel Cell CHP System

The low grade heat exchanger within a fuel cell system is able to provide pre-heating for

domestic hot water. Double wall heat exchangers are available from fuel cell manufacturers in

order to ensure that the potable water is not contaminated.

The output temperature of the domestic water from the heat exchanger will vary,

depending on the electrical load that the fuel cell is supplying. Data centers may not have very

many uses for domestic hot water other than restroom lavatories, service sinks, and break room

sinks. In this case, the full heat recovery from the low grade heat exchanger may not be

needed. Without the full use of heat recovery, the fuel cell’s overall efficiency will not reach the

maximum of 85% or 90%, as outlined in section 4.2.

For example, in the book “Combined Heating, Cooling, & Power: Handbook,” a hotel’s

CHP system with a 100kW fuel cell system yields 259,000 Btuh of low-temperature hot water

with a temperature ranging from 104ºF to 122ºF. If this system recovers all of the waste heat

(both high and low temperature), then the system can reach an efficiency of 89%. Without any

low grade waste heat recovered, the CHP system efficiency would drop to 60%. Even in a hotel

setting, where there are multiple showers, lavatories, service sinks, and kitchen plumbing

fixtures that require hot water, the author states that recovering all of the low temperature waste

heat would be difficult. Data center owners and designers alike need to be aware of these lower

CHP efficiency levels when determining whether or not a fuel cell CHP system will be installed

[55, p. 475].

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APC 400kW1MW User Manual Page 42