CPD PROGRAMME
Professional development
The CIBSE Journal CPD Programme
Members of the Chartered Institution of Building Services Engineers (CIBSE) and other professional bodies are required to maintain their professional competence throughout their careers.
Continuing professional development (CPD) means the systematic maintenance, improvement and broadening of your knowledge and skills, and is therefore a long-term commitment to enhancing your competence. CPD is a requirement of both CIBSE and the Register of the Engineering Council (UK).
CIBSE Journal is pleased to offer this module in its CPD programme. The programme is free and can be used by any reader. This module will help you to meet CIBSE’s requirement for CPD. It will equally assist members of other institutions, who should record CPD activities in accordance with their institution’s guidance.
Simply study the module and complete the questionnaire on the final page, following the instructions for its submission. Modules will be available online at
www.cibsejournal.com/cpd while the information they contain remains current.
You can also complete the questionnaire online, and receive your results by return email.
This module looks at energy efficiency in small-scale CHP systems Application of small-scale combined heat and power
Combined heat and power (CHP) can provide an efficient system by making both generated electricity and recovered engine heat for immediate local use. Through generating, and effectively consuming, heat and power simultaneously, CHP can reduce primary energy use by up to 30% compared to the more conventional methods, such as condensing gas boilers and grid supplied electricity. This CPD will concentrate on aspects of small-scale CHP, commonly referred to as ‘packaged CHP’, as the units are typically delivered to site as a complete item, ready for installation. CHP systems are commonly classified based on electrical output (kWe), as shown in Table 1. As shown in the example mini-CHP in
Figure 1, the engine known as the ‘prime mover’ drives a generator to produce electricity and recovers heat from the engine coolant, exhaust gas or – in the case of a larger steam turbine system – through spent steam. CHP effectively works as a localised power station close to the point of electricity and heat demands, avoiding transmission and distribution losses and utilising the waste heat locally, leading to higher fuel efficiency and potentially lower carbon emissions. Most ‘small scale’ CHP installations in buildings are packaged units, based on natural gas-fuelled reciprocating engines, with an electrical output normally less than 500 kWe. Small-scale gas turbines are also available that have lower
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maintenance requirements, but also lower electrical efficiencies. The primary energy saving for CHP may be compared1
to traditional systems by
considering the typical output from a CHP that is consuming 100 units of natural gas. This will generate 35 units of electricity and 45 units of useful heat.
Conventional grid-based electrical generation at 40% efficiency to produce 35 units electricity (40% = energy used at end user/energy consumed at power station) = 35/0.4 units of fuel = 88 units Conventional heating to produce 45 units of heat at 80% efficiency (80% total fuel efficiency of commercial heating system) = 45/0.80 units of fuel = 56 units The total fuel required, using a conventional supply to match the CHP output = 88 + 56 = 144 units
Micro has recently often been categorised as less than 2 kWe, as that ties in with the current maximum limit of the UK government’s ‘Feed in Tariff’ (FiT) funding for fossil-fuelled CHP.
* Table 1: Typical classification of CHP systems
So the potential saving in primary energy is about 30%. CHP can use many forms of fuel such as oil, biomass, bio liquid and bio methane, but the most frequently used fuel is natural gas.
The generator and electrical power production Asynchronous or synchronous generators are typically used to produce three-phase
Common classification Micro CHP
Electrical output <5 kWe*
Mini or small-scale CHP 5 kWe – 500 kWe Medium CHP
Large-scale CHP
500 kWe – 1 MWe >1 MWe
Figure 1: Packaged small-scale CHP (Source: Bosch) June 2013 CIBSE Journal 59
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