Sizing Combined Heat and Power (CHP) versus Sizing Standby Generators

by / Wednesday, 16 November 2016 / Published in CHP, COGEN, Technical Expert

Standby generators are necessary in many buildings as part of the Emergency electrical power supply and they are used to power a number of life safety devices.  Such devices include the fire pumps, fire alarm, emergency lighting, exit lighting, smoke evacuation and pressurization fans, and the elevators.  The emergency electrical power system along with the source, in this case a generator, and the equipment that it powers needs to be designed, installed, and must perform to meet the requirements of several different codes.  The list of codes primarily includes the Ontario Building Code, the Ontario Fire Code, the Canadian Electrical Code, Ontario Electrical Safety Code, and the CSA Standard for Emergency Electrical Power Supply for Buildings.

In one sentence these code requirements mean that when the power goes out the generator needs to get up and running to full output power very quickly so that the life safety systems can allow the building occupants to exit safely.  For many buildings this means that the power source needs to be sustainable for as much as two hours.

The required sizing of the generator needs to be able to support this two-hour operation for a number of lighting and branch circuit loads as well as several motor loads that all carry inrush starting amperage loads.  Selecting the right generator for your application requires that the block or step loading of the generator is managed in an orchestrated and controlled fashion to ensure that the following conditions are met.

  1. The generator must not be overloaded or under-loaded, typically minimum and maximum loading values are 30% and 90% respectively.
  2. The generator must provide sufficient starting kW and kVA to accommodate all of your motors.
  3. Whenever a step load is added to the generator the resulting voltage drop, frequency drop, and harmonic distortion all need to be within the tolerance as established by code and the equipment manufacturer for the application.

cogen1In order to achieve these sizing and operating conditions a large, powerful prime mover with a rich-burn fuel mixture and appropriately sized alternator are required, again to get the generator up to full power very quickly and able to run to provide power for a relatively short duration.  When compared to a Combined Heat and Power (CHP or Cogeneration) unit that is intended to operate continuously along with the utility power, this means that energy consumption and overall emissions are secondary concerns for a standby generator.

A CHP unit on the other hand should run as much as possible when economic conditions denote in order to provide the building owner with conservation and strengthen the return on investment.  As a result, CHP units have been optimized for energy efficiency and for extremely low emissions.  Unfortunately, in the case of several CHP manufacturers this comes with the sacrifice that they are either not capable of operating during a utility power outage, or that the ability to handle the step loading requirements of a building render the CHP unit unsuitable for use as a standby life safety generator.  The average CHP unit and a standby generator are two very different pieces of equipment.

While many CHP units are inadequate to provide life safety standby power or even secondary standby power there are many CHP products on the market that can accomplish both.  So should every building owner install a CHP unit to replace their standby generator when the time comes?  The answer is no, but for many building owners this might be a good solution.

To fully answer this question, we need to discuss some of the sizing strategies for a CHP unit.  Some common CHP sizing strategies include thermal load following, whereby the CHP unit is sized and operated to meet the thermal load of the building while the electricity generated during that time would be considered free.  The second strategy is to size the unit for the base electrical load, meaning the minimum electrical load experienced in the building, while the thermal energy would be considered the free “waste” product.  A third strategy is a hybrid between electrical and thermal load priority, which can be implemented such that the CHP maintains a minimum overall system efficiency, say 65% or higher where only a portion of the thermal energy is used and the balance is rejected to atmosphere.  Finally, a CHP unit can be sized to displace any portion of the existing electrical load from the base load up to 100% with little or no regard for use of the thermal energy where much of it would be rejected.  These sizing methodologies outlined above tend to produce increasingly large CHP systems but not necessarily increasing returns.  A qualified energy analyst or engineer should be consulted to determine what sizing strategy is most suitable for you and your building and what the associated return on investment would be for each strategy.cogen2

Then if the size of CHP unit that a building can reasonably support, as a result of the building’s electrical and thermal utility profiles and chosen sizing strategy above, matches or exceeds that of your life safety standby generator plus if a suitably sized CHP product meeting all your criteria for use as a standby generator exists in the marketplace this is a great solution for your building that will maintain the safety of your building and its tenants as well as provide a return on investment for the owner.  One further caveat is that installing a CHP unit fully integrated with the building’s electrical and thermal systems will cost on the order of 8 to 10 times what a simple like-for-like replacement of a standby generator would be.  Therefore, the decision should be deferred to the economics involved for each option and how the local utility company will let you operate your CHP.

Where the sizing and economic analyses suggest that using a CHP for double duty as an emergency standby generator would be a poor choice, all is not lost.  A CHP unit can still be installed to operate in parallel with the utility and produce an attractive return on investment.  Alternatively, a secondary standby power distribution arrangement can be designed to provide standby power to equipment that is not considered life safety but yet the physical building and tenants would gain a significant benefit from having additional equipment supported by a means of backup power.

As you can see sizing a standby generator is very different from sizing a CHP unit and the processes of sizing and evaluating the code requirements and economic synopsis of each are quite involved and complicated.  A qualified and experienced expert in the breadth of topics discussed in this article should be consulted to determine the best approach for your buildings and goals for the asset.

Mike Zongor, P.Eng.

Mike has extensive experience in energy auditing and design. Since joining the company in 2004 Mike has personally developed, designed, and managed the implementation for the retrofit of over 220 boilers, 25 chillers, 50 air-handling units, as well as the installation of major upgrades of over 250 building automation systems.

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