Top tips for specifying a combined heat and power system

    There is growing interest in combined heat and power (CHP) as a viable alternative to traditional renewable energy supplies. David Hatherill, engineering manager at Finning Power Systems, UK, describes two CHP projects that the company has been involved in and advises on effective selection and specification of a CHP system.

    By: David Hatherill, Finning Power Systems, UK

    Combined heat and power, sometimes called cogeneration, is the production of heat and power in a single process. Electricity generation produces a large amount of heat as a byproduct. In traditional power generation methods this is wasted, but CHP captures this heat to use in other areas.

    This reduces the generation, transmission and distribution losses that occur when fuel is burned remotely in a central generation facility. Industrial, commercial and some public sectors are the main users of such schemes, which range in size from a few tens of kilowatts to multiple tens of megawatts. Good examples of applications of CHP are those that require a great deal of heat, such as hospitals, universities, leisure centres, offices, wastewater treatment facilities, chemical plants, large glasshouses and retailers.

    For the Norfolk and Norwich PFI Hospital, UK, for instance, Finning supplied a power generation package that comprised a CHP system and a separate 11 kV diesel standby, both in a stand-alone energy centre. A 1 MW reciprocating gas engine powers the CHP system that provides power and heat to the hospital’s heating and ventilation medium-pressure hot water ring.


    Finning LIMA control system with colour touch-screen interface
    Click here to enlarge image

    The prime mover is a Caterpillar G3516 low-emission, lean-burn gas engine. Heat from the engine exhaust, jacket water and oil cooler circuits is recovered to provide 1314 kW. This heats up the ring’s returning medium-pressure hot water before it re-enters the boiler, so the CHP system acts as lead boiler. When thermal demand is low, the excess heat is dumped to a remote radiator. Finning’s LIMA remote control and monitoring system controls the whole process, with a control panel providing a graphical overview of the complete CHP system. There is no keypad; operators use the colour touch-screen interface. In addition, the panel can autodial the Finning 24 hour control desk if there is a problem. Many issues can therefore be diagnosed and rectified, and the machine then restarted without the need for a technician to visit the site.

    In a different but equally successful application of CHP, Thames Longreach sewage plant in the UK uses three CHP biogas generators for the production of up to 3.3 MW per hour of green electricity. The generator sets use the methane gas produced by the sewage digestion process as the fuel gas. The resulting electricity production powers the sewage operation, offsetting electricity generated by fossil fuel. Any excess production is exported to the grid. Each megawatt exported to the grid is enough to power 1200 homes.


    One fo the biogas generator set at Thames Longreach sewage plant in the UK
    Click here to enlarge image

    Double skinned expandable balloon gas holders store the methane gas before use. This means that the amount of fuel used, and therefore electricity generated, can be more effectively managed. A back-up diesel generator set provides failsafe power generation if both of the existing biogas generator sets are unavailable. Another auxiliary diesel generator provides a third level of back-up.

    Again, the whole process is managed by a Finning LIMA control system. This can switch between generator sets automatically and also prioritize which circuits are powered if there is a problem. Two levels of redundancy mean that the generator sets can be switched automatically or manually using LIMA, or manually using key switches. Once again, LIMA allows remote monitoring of the whole system as part of Finning’s 24/7 maintenance contract.

    The CHP system also generates 2 MW of heat, which raises the temperature of the sewage digestors to generate more methane gas for power generation. Not only is this a neat CHP solution, it also makes use of a potent greenhouse gas, making it a truly green solution.

    Both of these applications have proved highly successful and illustrate how CHP can be applied in diverse situations.

    CHP’s golden rules

    However remember, a CHP system is a sizeable investment, so specifying the correct system is important. Mistakes at this stage can be costly. So, in our experience, what are the golden rules for specifying a CHP system?

    1. Know your annual heat and power requirement

    CHP is ideal for buildings requiring both heat and power for more than 4000 hours a year. A hospital, leisure centre or factory, for example, is likely to meet these criteria, whereas a school is not.

    2. Know your electrical load profile

    It is more important to understand the baseload profile rather than the mean or peak. Baseload is the minimum continuous electrical demand over the entire period for which you are considering CHP. Getting this figure right and then making the correct selection will ensure that the system runs efficiently by maximizing utilization and plant conversion efficiency. The best way to identify what is going on is to plot load continually against time on a graph and to look at this over a year (see Figure 1). Profiles over a number of typical days will also be needed.


    Figure 1: Typical electrical load profile
    Click here to enlarge image

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    3. Know your heat demand in kilowatts and degrees Celcius

    CHP configurations vary widely according to the amount of heat required and the temperature that is required. Some engine manufacturers will offer engines with much higher jacket heat recovery temperatures than others. It is important to know the temperature at which heat energy is required because some low-grade heat cannot always be recovered. Heat available at 50 à‚°C may be suitable for chillers and heating swimming pools, for example, but may not be adequate for CHP in a hospital. Remember that a CHP system can not only be used to keep facilities warm in the winter, but the waste heat can also be used to run absorption chillers in the summer to keep buildings cool.

    4. Know how much you currently pay for heat generation using gas or oil & electrical power without CHP

    The financial benefit of the generation of CHP is not solely the cost savings accrued from improved efficiency. By using current billing figures, a precise cost comparison can be produced that shows the potential savings that each CHP system will provide. There are many tariff options. In some cases, CHP can add significant revenue in times of high external demand for electricity on the supply network. Often, a lucrative feed-in tariff and/or a ‘set-aside’ payment for not drawing down power from the grid during these peak periods can create a considerable income for a CHP generator.

    5. If in doubt, undersize

    A CHP system that is too large for the application may not save money as it will not be able to run efficiently or its utilization will be low, whereas a CHP that is too small will save money, but slightly less than it could had it been correctly sized. It is a good idea to size the unit so that all heat and electricity can be used, and the unit always runs at full capacity. Remember that neither heat nor electricity is easy to store.

    6. Understand life-cycle cost

    The cheapest CHP solution identified during the procurement process will not necessarily be the cheapest to run over a period of time. CHP systems have complex control systems, and the more expensive packages tend to have higher functionality and better quality components. These generally have reduced down time, greater savings and better long-term value. Bundling CHP with a main contractor’s build contract generally does not result in the best solution for the end user.

    7. Track record.

    There are many existing CHP systems that under-perform, so look for a supplier with a good track record. Reliable suppliers will always allow you to view examples of their work, explain how and why it saves money and introduce you to long-term satisfied customers. It is also worth checking that the supplier has experience of building CHP systems in similar applications to your own.

    8. Buy an operations and maintenance (O&M) service contract

    Do this at the same time as the installation. This will guarantee the system’s performance, reduce risk and staffing costs, and can remove the need to budget for servicing, maintenance and overhauls. Such contracts can be rolled up into a fixed monthly charge indexed to RPI for easy budgeting. Leasing schemes are also available, with the customer simply paying for heat and power.

    9. Listen to the supplier

    Reputable suppliers understand where money needs to be spent and where you can economize. While you may want cost-effective CHP, it also needs to perform well. Beware of specifications prepared by anyone who does not own, build, operate or maintain CHP systems. In addition, suppliers with experience of your industry may have suggestions that you did not know were possible. Don’t start with a pre-engineered solution. Good CHP is nearly always custom built.

    10. Know which renewable payments you can apply for

    The viability of CHP can vary greatly according to these payments. There are many incentives that make CHP attractive. It is important to know which ones you can apply for at the time of purchase as the energy metering of the project will need to be installed according to their qualifying guidelines. Visit the Renewable Energy Association website for more information (www.r-p-a.org.uk).

    Good quality CHP may not be as ‘clean’ as renewable energy, but as it is usually a gas fired product, it is cleaner than many other systems fired by fossil fuels. It is efficient because it combines electricity and wasted heat, thereby producing a 3:1 improvement in the emission of CO2 into the atmosphere. For an even greener solution, it is possible to combine CHP with the use of fuel such as digestor gas or landfill gas. Thames Longreach, described earlier, is a good example.

    This gives all the benefits of CHP, does not use a fossil fuel and, better still, by using methane from the sewage prevents a harmful greenhouse gas from reaching the atmosphere.

    CHP is a viable alternative to traditional renewable energy supplies. For the right installation it can make a lot of commercial sense to install such a system, but it is important to specify the correct system. Getting it wrong can prove costly, so make sure that you explore your options with an experienced and proven organization.

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