Combined heat & power
When it comes to return on investment, size matters
In buildings where there is a large heating/cooling demand over extended periods, the benefits of combined heat and power (CHP) are hard to beat. This applies equally well to new build or retrofit projects. It is an ideal choice when looking to replace existing boiler plant or as an addition to new or existing boilers, says Ian Hopkins
circumstances and when sized accurately, CHP can yield a typical return on invest- ment within three to five years - providing impressive cost savings over a typical 15 years+ product lifecycle. Failure to size your CHP system correctly, however, will cancel out the benefits of choos- ing CHP.
Investing in a CHP plant is simi- lar to leasing office space or build- ing a manufacturing facility. Obtain too much space and you end up paying for more than you need. Invest in too little and you either lack enough to reach your full potential as a business, or you have to add on or find new space later. According to the Chartered Institution of Building Services Engineers (CIBSE) the typical cost of installing a CHP is between £600 and £1,500 per kWe (depend- ing on the unit size). The size of the investment means it’s crucial to achieve maximum return. This means installing a CHP plant requires careful deliberation when determining the optimal size. A plant needs to operate as many hours as possible, since idle plants produce no benefits. A CHP engine cannot run below a minimum load. If it’s too large, it will not operate enough. A system that’s too small will not provide the full cost savings. Poorly sized systems will not per- form optimally.
CHP preparation
Before installing a CHP into an existing structure, it is worth opti- mising a building’s energy enve- lope. Consider other efficiency measures first. Better insulation, staff training, and utility buying should be explored before prepar- ing a plant for a CHP installation. Once this has been done, you should obtain data for electric and heat demand to accurately size the system. It is advised that you go as far as using hourly demand data to determine the actual amounts of heat and power that can be sup- plied to the building. It might be
24 | July 2014 | HVR
beneficial to install temporary metering or monitoring equipment to establish heat and power demands in detail.
In new buildings heat and power demand profiles can be estimated using a combination of: building design data; simulation modelling of building; benchmark profiles from comparable build- ings; occupancy patterns, and data from energy models.
It is important to take into account other energy efficiency measures. You should also consid- er any future changes in energy
requirements such as a reduction in heat or power demands. By establishing a detailed model of the heat and electrical demand, you can then establish the size of your CHP plant based on the following considerations:
1. Baseline For optimal efficiency, CHP units should be designed to provide baseline electrical or thermal out- put, with any shortfall being sup- plemented by electricity from the grid or heat from boilers. In certain cases there is the option to size
slightly above the thermal baseline to deliver higher electrical output and greater financial savings. At times when the CHP output exceeds the thermal demand there is a need to reject heat. This is achieved through the operation of a dedicated dry air cooler or cooling tower. Getting rid of excess heat enables the CHP unit to maintain its full electrical output but would reduce its efficiency. Therefore, a careful balance should be achieved between CHP size and site demand.
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Inside the ENER-G CHP unit at award winning LifeScan site
Ian Hopkins sales and marketing director ENER-G Combined Power
CHP can yield a typical retun on investment within three to five years
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