Figure 2: Potential peak operating efficiencies for mini-CHP (Source: Bosch)


electricity at 50 Hz 400 V for connection into the low voltage site distribution system. The generated power would normally be used locally. It is possible to export power into the grid with agreement from the local district network operator (DNO), but for natural gas fuelled mini-CHP module, this is generally uneconomical – particularly as the price paid by the networks could be just half of that paid for buying in electricity. When establishing the potential benefits of applying CHP, the effect on the mains electrical supplier must be considered, as both peak electrical demand and overall site consumption will reduce, affecting the negotiated tariff costs.


The potential for CHP At the end of 2010, there were just under 1,200 non-industrial CHP installations in the UK2


,


and the introduction of UK government’s ‘Feed in Tariff’ (FiT) payments (guarantees to pay a fixed tariff for each kWh of electricity generated, and an additional payment for each kWh of electricity exported to the grid) has accelerated the adoption of micro-CHP systems, with more than 4003


having been installed since 2010. The


trial undertaken by the Carbon Trust (reported in 20114


) of 87 micro-CHP installations in


typical UK households and small commercial applications, found that the larger domestic systems – using more than 15 MWhth heating per year –and small commercial premises were most likely to benefit from savings in carbon, with average CO2


savings of up to 16%. On


average, small domestic installations benefit (in carbon terms) by a few per cent compared to using a condensing boiler and grid electricity. The first three years of the UK government’s FiT has provided an incentive for micro-CHP (<=2 kWe), helping to boost the adoption – and expertise in installation – of small gas-fuelled units. This is likely to improve the performance of future installations. Using factory produced small-scale CHP schemes may potentially achieve peak operating efficiencies of up to 90%. The seasonal efficiencies will be dependent on the matching


60 CIBSE Journal June 2013


of the year-round local thermal load to the heat produced by the CHP. Applications that are particularly suitable are those with consistent heating and/or hot water loads alongside electrical demand, such as hospitals, leisure centres with swimming pools, and factories with processing requiring heat/hot water/low pressure steam or district heating systems.


Economic and environmental considerations The CHP is powered using a lower cost fuel, such as natural gas, to produce energy with a higher value in the form of electricity. The difference between the cost of these two fuels (known as the ‘spark-gap’) has a most significant effect on the economic viability of a project. As the spark-gap increases, the payback period for the capital (and operational) investment will reduce. Manufacturers quote a product life of around 10-15 years of operation (with appropriate maintenance), and although fuel prices will change over that period, the separate fuel prices typically increase together to maintain a practical spark-gap. To provide increased benefit, a CHP scheme


should operate as many hours per year as possible. As a general ‘rule of thumb’, the


recommended minimum running hours for financial success is an equivalent of 12 hours per day, with well-selected, designed and operated systems potentially providing payback within five years. In the UK, the heat demand is inevitably seasonal. However, by using heat-driven absorption chillers, there are opportunities to extend the season so that heating base load is increased during the summer months, enabling a more effective use of the CHP plant. Symbiotic energy-linking with nearby users that have complementary heat/power profiles can provide highly effective solutions – for example, a tomato farm and supermarket, or a university administration block and student residences. Apart from a potential reduction in carbon emissions, there are a number of financial incentives in the UK to encourage the application of CHP systems. Natural gas-fuelled systems with an electrical


output no greater than 2 kWe are currently eligible for funding under the FiT scheme.5 Systems (up to 5 MWe) fuelled with biogas from anaerobic digesters are also eligible for FITs, as well as the Renewable Heat Incentive (RHI) for up to 200 kWth.6 The Enhanced Capital Allowance (ECA)


scheme allows businesses to claim 100% first year capital allowances on investments in qualifying energy saving technologies and products that can include CHP. The Quality Assurance for Combined Heat and Power (CHPQA7


) provides a standardised


method to compare the effectiveness of CHP systems, through a quality index (QI) and power efficiency. This provides the principal evidence required for determining eligibility of CHP schemes for ECAs – see CHPQA document Guidance Note 42 – and Climate Change Levy (CCL) exemption. Qualifying CHP projects can be exempt from


Some installation factors Noise – The CHP can generate significant engine noise and vibration that may transmit through the pipework and the exhaust/flue systems. Packaged units will often be supplied with attenuated enclosures, although additional silencers and vibration isolation may well be required. Access – Installation will require appropriate access (often including cranage), but the future maintenance needs of engines requires significant clearance. Flue/exhaust system – These need to be high- pressure systems (potentially operating with 5 kPa pressure), with gas velocities of 25-35m·s- at high temperatures.


¹


Hydraulics – Care must be taken with the design/installation/commissioning of the water side of the systems CHP, otherwise it may cycle more frequently, leading to higher maintenance costs and longer payback periods. Ventilation – Casing losses from CHP equipment will require ventilation (particularly in summer periods). If the CHP combines a heat extraction system, extra ventilation will be required to maintain a positive pressure inside the plant room. Grid connection – Contact the district network operator (DNO) as soon as the project starts, as they will be able to identify what is required to meet regulations.


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