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

In the rIght circumstances and when sized accurately, CHP can yield a typical return on investment 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 choosing CHP.

Investing in a CHP plant is similar to leasing office space or building 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 (depending 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 perform optimally.

CHP preparation

Before installing a CHP into an existing structure, it is worth optimising a building’s energy envelope. Consider other efficiency measures first. Better insulation, staff training, and utility buying should be explored before preparing 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 supplied to the building. It might be 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 buildings; occupancy patterns, and data from energy models.

It is important to take into account other energy efficiency measures. You should also consider 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 output, with any shortfall being supplemented 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.

2. Load

Following CHP units have the ability to modulate, or change their output in order to meet fluctuating demand. These CHP units can be set up to track either the electrical or thermal demand profile. The decision to track thermal or electrical load depends on the heat to power ratio of the site and associated energy costs. When following the electricity demand, the implications of possible heat dumping into the atmosphere via heat trim or heat dump radiators have to be fully analysed.

3. Electricity export

Another way to deal with excess electricity is to export to the power grid, however this must be carefully evaluated as it can have significantly lower value than electricity consumed on site. Another strategy is to employ multiple CHP units instead of one larger one. Using the former strategy, an operator would set up a series of units to cascade to meet energy demand during times of peak demand. One unit would meet the baseline while smaller plants would provide excess needs.

Load duration curve to size CHP

To correctly size a CHP, an organisation must understand how long a particular demand exists for. A very effective way of assessing load is by producing a load duration curve.

This load duration curve shows heat load for one year and how two CHP units can be used in conjunction with each other. One 180kWe unit operates for 6,000 hours each year, and the other 90kWe one runs for 5,000. Load duration curves can also be used very effectively to analyse a site’s existing electrical demand.

Reaping the benefits of CHP

Since installing a gas-fired ENER-G cogeneration system in April 2012, medical device manufacturer LifeScan has reduced its carbon footprint at its Inverness site by 625 tonnes per year. This is equivalent to the environmental benefit of removing more than 200 cars from the road or of the carbon dioxide sequestered annually by 512 acres of forest.

LifeScan Scotland, a Johnson & Johnson-owned company, manufactures blood glucose monitoring devices and the Inverness facility is a key research and development location for the global group.

M&E building services consulting engineers Hulley & Kirkwood was commissioned by LifeScan to carry out the initial feasibility study to reduce overall carbon emissions and to cut energy related costs. Following its report, CHP was considered to be the most viable and efficient solution with the greatest carbon saving, and ENER-G was selected to undertake the CHP project.

After analysis and detailed specification, ENER-G designed, manufactured and installed a 230kWe CHP system, which has been integrated with the site’s boiler network system to supply the manufacturing facility with low-temperature hot water, as well as power. The system is fully containerised and includes roof-mounted dry air coolers.

Carbon emissions at the site have reduced by 21 per cent, and electricity consumption is down by more than half (53 per cent), lowering the energy bill by 27 per cent. As a large site, LifeScan is subject to the CRC Energy Efficiency scheme and the CHP system is making a substantial contribution towards its CRC performance.

LifeScan recognises the need for regular CHP maintenance and has taken out a fully comprehensive operation and maintenance package with ENER-G. This provides a range of services, including 24-hour remote monitoring, a dedicated site engineer and all inclusive parts and labour required to rectify faults or repairs for the contract term.

Following the success of the CHP installation and the high performance and savings achieved, LifeScan took the decision to purchase another CHP unit from ENER-G, which has been installed in the Inverness site’s phase 3 building.

Jason Whitley, LifeScan’s facilities senior project engineer, said: “We are continually seeking ways to raise our environmental performance and this move to on-site generation of power is a key element of our carbon cutting strategy. We are very pleased to be partnering with ENERG, which is able to provide us with a total service – from initial design to long-term care of the systems – ensuring our commitment to a sustainable future.

// The author is the sales and marketing director at ENER-G Combined Power Ltd //