Adding the benefits of CHP to existing buildings
The realities of installing new CHP plant into an existing building — one of two 1.6 MW(e) units for the Pimlico district-heating scheme is lowered through a tight hole into the plant room.
Heather Jefferson explains how best to exploit the energy-saving potential of CHP plant in existing buildings to produce power, heating — and cooling.Reports on the environmental consequences of excessive energy consumption, both at home and in the workplace, are currently receiving a spectacular amount of column inches and media air time. Reducing our carbon footprint is the current media hot topic — partly driven by an increase in utility prices and partly by new legislation pushing society towards increased energy efficiency. A great example of this new legislation can be found in the building industry, where any new developments must conform to Part L of the Building Regulations. There are various ways in which Part L compliance can be achieved, but the installation of a combined heat and power (CHP) unit will make a significant contribution to reducing a building’s carbon footprint. One important thing to remember, however, is that CHP is not just technology for new building projects. The installation of a CHP in an existing building can offer exactly the same benefits to the end user. Before discussing the benefits of CHP it is important to ensure that the proposed site is suitable for CHP. Selection criterion One of the best-known selection criterion for CHP is the year-round heat demand. Sites such as leisure centres and hospitals have been proven to work well with CHP due to their constant demand for heat. If the proposed site does not have a significant heat demand during the summer, there is the possibility that the site may instead require a cooling system. If this is the case, a trigeneration or CCHP (combined cooling, heating and power) scheme incorporating a CHP unit and an absorption chiller may be considered as an effective solution. Once the site’s suitability for CHP has been established (most CHP suppliers offer free feasibility studies) the next step is to size the CHP. This involves trying to select the right CHP by achieving a balance between meeting the heating demand and/or rejecting some heat, whilst still achieving savings and meeting the criteria for good quality CHP. When the CHP has been selected, the next question is usually cost. The cost to retrofit a CHP can be affected dramatically by certain design issues. The first decision to be made is where the CHP is to be located. This decision will pose many questions such as whether there is enough space for maintenance access and whether there are noise and/or vibration constraints. Once all the constraints have been assessed, the mechanical and electrical connections should be considered. The heating system will normally be set up with CHP as the lead boiler, with the site boilers providing a top up of heat to meet peak demands. The location of water and gas connections can add to the cost of pipework and must therefore be given due attention when calculating overall installation costs. The other mechanical installation aspect to consider is the ventilation air, combustion air and exhaust. The CHP supplier will assist with ductwork specification, and a collective discussion is then required regarding the routing of the exhaust. Electrically the CHP is usually connected to the low-voltage (LV) distribution network, with the national grid supplying any additional demand required by the site over and above that supplied by the CHP. The location of the LV panel needs to be determined to calculate cabling and installation costs. Once the design issues have been resolved and installation costs established, the feasibility of the project then has to be determined. When assessing the feasibility of the project, there are two main finance options to consider —capital purchase or a supplier-financed scheme (some of the CHP providers offer this option). If the CHP is installed under a supplier-financed scheme, the funds allocated for the CHP could then be used elsewhere. It is possible that an end user may have been awarded a grant or specific budget funding to refurbish the building plant. Finance When considering finance options, a supplier-finance package usually takes care of the maintenance — giving the end user total peace of mind. If the CHP is a capital purchase, it will need regular maintenance based on its operating hours. This maintenance can be carried out by the CHP supplier as part of an agreed maintenance contract or on an ad-hoc basis. A trigeneration (CCHP) project will need to include a further set of considerations — such as calculating the cooling demand, finding a suitable location for the absorption chiller, the cooling tower or the adiabatic radiators and a review of the connection and installation issues as with a typical cogeneration system installation. There are many benefits for both the end user and building- services professional when considering CHP or CCHP. However, each application needs to be carefully assessed on its own merits. It is important to involve the CHP supplier in the early stages of the project so that the feasibility can be assessed and the benefits highlighted. The typical benefits of a CHP installation can be summarised as follows. • Complying with legislation. • Reducing carbon-dioxide emissions. • Security of supply. • Financial savings. • Providing the opportunity to claim Climate Change Levy. • Enhanced Capital Allowances and other project-specific benefits. Heather Jefferson is with Ener-G Combined Heat & Power Ltd. This article is based on a recent seminar on retrofitting combined heat and power organised by the Chartered Institution of Building Services Engineers. [Ener-G has over 20 years’ experience in CHP — with more than a thousand units manufactured, some 5700 running years logged and over 1.6 Mt of carbon dioxide saved by CHP users. Ener-G has produced ‘The essential guide to small-scale chp’. To obtain your free copy call 0161 745 7450 or visit the web site below.