Putting a Power Plant in the Basement: Residential CHP

by ‐ Tags: alternative energy, green living

Today we have a guest post from Shiva Prakesh, one of our friends at New Ecology.
In the world of green technologies, you may have been hearing more lately about cogeneration systems, also known as combined heat and power (CHP) systems. CHP systems have been around for over 100 years, with the first system being implemented by Thomas Edison in 1882. 

Cogeneration systems and CHP are terms that describe a broad category of systems that essentially capture heat byproducts from electricity production and use it for space or process heating. Normally, this waste heat is simply rejected and not utilized. With CHP, you are capitalizing on electricity generation and saving energy making it an increasingly popular technology given rising energy costs.

The scale of these types of systems ranges quite a bit. The first examples of cogeneration were on large scales, either in industrial factories or as a part of district-wide systems. But recently there has been a lot of development in mini/micro cogeneration systems that serve a single building as well.

Here is a good schematic that summarizes how a micro CHP system works:

CHP

Micro/Mini Combined Heat & Power
Utilizing a micro CHP system in a home or small residential building can have energy and cost benefits.  

Building-level cogeneration systems (micro CHP) usually burn natural gas to run an electrical generator. The combustion of the natural gas creates the waste heat that can be used for space heating. Given that natural gas prices are relatively low, particularly compared to electricity costs in many areas, this type of system can save a property or homeowner money.

There are also alternative fuels that can be used to run some CHP systems, such as biomass  (like wood chips), though this is less common than the natural gas generators. In residential CHP, the waste heat can be used to warm water for circulation through hydronic heating systems and could have the capacity to provide heating of domestic hot water as well. However, there are a number of important considerations when sizing and designing the system that will ultimately determine how efficient the system will be for a homeowner.

Sizing the System
One significant issue that arises with CHP systems in residential settings is that of correct sizing, to avoid either over-production of electricity or waste heat. Say the system was sized to meet the space heating demands of a house in the Northeast, it is likely that at least some of the time, the electricity production required to generate sufficient space heating will be in excess of the electricity the house actually needs. One solution is to undersize the system to meet the minimum load of heat or electricity, but if the system is undersized too much, the homeowner will lose much of the cost and energy benefit of installing a CHP system in the first place. So it is important to have measures in place to handle excessive production of the heat or electricity in a CHP system.

Dealing with Excess Electricity
In the case of sizing the system such that there will be excess electricity at times, this can be handled with net-metering, where excess electricity can be fed back into the grid and the homeowner can actually receive credit for this, just like a net-metering scheme with a rooftop photovoltaic system. Unfortunately, net-metering in general is highly regulated and in many states only particular types of systems will be considered for net-metering, so there are places where this is not a viable solution. An alternative if net-metering isn’t available is to run an electric resistance heater that will take the excess electricity and turn it back into space heating. This might make sense in a setting where the system has been designed to meet a high space heating demand, and is therefore producing more electricity than is needed for the electric load, but this electricity can then be used to assist with meeting the overall heating demand of the building.

For more information check out this (very detailed) report: report for the Department of Energy on a range of cogeneration systems and issues. For a lighter read, check out one homeowner's experience with micro-CHP.

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