Overview 

The overriding purpose behind installing a CHP unit is take financial savings and deliver emission reductions. With the high fuel efficiency of CHP plants, the carbon dioxide emissions per kWh of electricity or heat generated are relatively low. A well-designed and operated CHP plant will therefore improve energy efficiency delivering cost savings and significantly reducing carbon dioxide emissions. CHP is a process in which both heat and electricity are produced simultaneously by an on-site prime mover. The heat generated when the fuel is burnt (typically natural gas) to produce electricity is captured and utilised for processes such as space heating, domestic hot water heating or refrigeration (via absorption cooling). Typically this achieves a 35% reduction in primary energy use

The Technology

 In a similar manner to most other means of generating electricity CHP plants use a prime mover to drive the electricity generator. CHP units can be based on a variety of prime mover technologies: ü  Gas turbines – these drive a turbine generator and produce exhaust gases at between 400 and 550ºC. Gas turbines are typically available as 0.5 to 200MWe systems and produce high grade heat.ü  Reciprocating engines – these are fuelled by gaseous fuels. Spark ignition engines are available up to 4MWe and compression ignition engines (diesel engines) up to 15MWe. High-grade heat at about 400ºC is available from the exhaust gases from reciprocating engines. Low-grade heat at about 80ºC is available from jacket cooling and lubricating/cooling systems.ü  Steam turbines use any fuel to generate steam and systems and are available from 0.5MWe upwards. Steam turbines are often used in conjunction with waste heat boilers and produce medium grade heat and can incorporate several pressure stages.  

CHP Suitability 

CHP is not an all-purpose solution and its suitability needs to be checked carefully.  The key factor in the decision to install CHP is the ability to use the heat generated by the CHP system across the year. In order to be cost effective the system ideally needs to run for a minimum of 5,000 hours/year at equivalent fuel load. For this reason the suitability of CHP for a particular process or application requires a detailed appraisal of heat and electricity requirements, along with other factors. A feasibility study should be carried out on a site-by-site basis CHP is only generally cost-effective where the heat loads are present for at least 12 hours/day, 7 days/week but in general, the greater the annual period of demand, the higher the cost savings.  There should be also reasonable confidence in the future demand for heat and power over the lifetime of the CHP plant, typically 10-15 years.  There must be an adequate supply of a suitable fuel for the CHP unit which has a considerable greater than fuel requirement than a boiler plant of similar size.  Regulations also require a CHP to be flued separately from a boiler and suitable acoustic and vibration attenuation measures need to be taken as well as access for the future maintenance/removal of the CHP plant.

The spark gas or ratio between gas and electricity prices is a key financial driver in the determination of whether to proceed with a CHP project, however the capital costs if important, particularly the capital cost per kW of output. Typically the payback on investment in CHP is more than five years.  

Financial Incentives 

There are financial incentives for installing CHP, which come in addition to saving energy costs.  Through the CHPQA scheme the Government has put in place three main incentives to encourage the take-up of 'good quality' CHP:

ü    Investment in a CHP plant may qualify for an Enhanced Capital Allowance (ECA). This enables a business to claim 100% first-year capital allowances of their spending on qualifying plants and machinery. Businesses can write off the whole of the capital cost of their investment in these technologies against their taxable profits of the period during which they make the investment.

ü    Exemption from the Climate Change Levy.

ü    Exception of Power Generating Plant & Machinery from Business Rates. 

Micro-CHP

This technology which is not widely utilised in the market at this time for a number of reasons including: 

ü Cost per kW

ü Size of unit

ü Overall efficiency

ü Balance of heat to power loads in a residential dwelling 

is a miniature CHP unit designed to provide heat and power to a small building/domestic dwelling in place of a conventional boiler. An example of the technology used in a micro CHP unit is a Stirling engine. If a number of these issues can be overcome then uptake may increase over time.