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ACHIEVING CARBON REDUCTION TARGETS CHP or Solar PV: Choosing the best technology
The choice of technology used to achieve carbon reduction targets is an interesting and important decision for any building services engineer. In this article, Adveco weighs up the advantages of CHP (Combined Heat & Power) when compared with Solar PV systems.
reductions, but must also incorporate factors such as the system capital cost, longevity, and degradation of efficiency over time.
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From the client's perspective, there are also savings and costs after commissioning to consider, for example, incentive schemes and routine maintenance. With turbulent energy prices, changing incentives, and the influx of new technology to the market, it is very important that designers regularly review their technology choices; engineers can no longer incorporate a default technology into all designs and still achieve a satisfactory result. Two popular technologies available are CHP and PV. With recent changes and the general long-term uncertainty of incentives, CHP may be the better option.
he best engineered solution must not only meet the required carbon
must consider the times that a building will be occupied, and the electrical demands while occupied compared to those while unoccupied.
The occupancy of many buildings is significantly reduced over the weekends and holiday periods which can equate to 30% of the available time. Schools, as an example, typically operate at reduced levels for nearly 50% of the year, including the summer months when solar output is at its peak. When PV is sized to meet a building's full carbon reduction target, it is likely that the electrical demand will, at times, be lower than the PV output. Such periods increase the payback period away from the ideal 7 years and towards 13 years.
array and directly affect the suitability and effectiveness of PV.
Conversely, buildings with limited plantroom areas may not be best suited to CHP systems that require thermal storage. The construction of the building therefore has a direct impact on the installation costs and energy contribution which determine the payback period.
uThe TOTEM micro‐cogenerator is the first in a new generation of high efficiency, low emissions CHP. This compact and reliable combined heat and power unit has what is considered to be the highest efficiency and lowest emissions available on the UK market.
uA Solar PV installation. This technology is appropriate for buildings with high summer electrical demands, for example, to power air conditioning in offices.
The review of the Feed-in Tariff scheme in 2015 had a dramatic effect on the economic value of PV solar systems. The FiT was previously so valuable at 9.5p/kWh that it was worthwhile regardness of whether the electricity was used on site or exported to the grid. A 100kW system could reach a 5-year payback if all the electricity was used on site.
If it was all exported, the system could still achieve a 7.5-year payback when combining the FiT with the export tariff. This allowed designers to simply calculate the number of collectors required to meet the yearly carbon savings, and the system would be cost effective for the client regardless of the building’s electric demand to PV peak output ratio.
Following the consultation, the FiT payments fell by 80% for 50-250kW solar PV systems to <2p/kWh, making proper analysis of site electricity consumption very important. The value of displaced electricity – in terms of overall energy costs – is now 5x that of the FiT, and 2x that of exported electricity. Using today’s rates, the same system using all electricity on site will achieve a payback of 7 years; but when fully exported, the payback period could stretch to 13 years.
It may not be in the client's interest to simply match the PV output to the necessary carbon savings; designers
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When payback periods exceed 7 years, PV systems should be compared to CHP to reduce the building's carbon emissions by an equivalent amount. A typical 100kW PV system can reduce carbon emissions by 46,000kg. Similarly, a 25kWe CHP unit operating for 7500 hours/year would achieve an equivalent net emissions reduction. It is assumed for the CHP system that all the electrical output, which has much lower peak magnitude than PV, can be used on-site. When correctly designed to achieve over 5000 run hours, this CHP design can deliver a payback period of between 6.5 and 10 years.
Since both CHP and PV can be used to achieve equivalent carbon reduction, the financial case for each should be considered. Maintenance costs for both systems are accounted for in the payback ranges, and long- term repair costs can be similar over 25 years. A key advantage of CHP is that it can be periodically overhauled to restore efficiency to its peak value, whereas PV systems will experience irreversible degradation over time. The range of payback periods are similar for both systems, so a building’s characteristics will determine which technology is more suitable; empirically, it is likely that one system will provide a better payback. The period is affected by the installation and the energy demands of the building.
The installation is governed by the physical size, construction, and location of the building. Planning constraints, building orientation, and roof area may limit the location and size of a solar
The energy demands of the building will suit one technology more than the other. CHP only offers value when it runs, and it can only run if there is a thermal demand. CHP systems are therefore more suited to buildings with year-round heat demands. Residential buildings with significant DHW usage, such as hotels, university
accommodation, prisons, and care homes fall into this category, as do buildings with fixed loads such as leisure centres.
Solar PV now offers the most value when the building consumes the produced electricity. Since it produces more electricity in the summer, it is the most appropriate choice for buildings with high summer electrical demands, such as air conditioned offices and retail. With the current Feed-in Tariffs, both PV and CHP systems can provide an equivalent return on investment, but one is likely to stand out as the best choice for each individual building. Consideration should be given when designing to the uncertainty of PV incentives for long-term projects and the associated risk to the financial case if changes occur before completion of the building. Many projects are more suited to use CHP than PV and with sufficient analysis, the design of the building itself should guide designers to the technology required to achieve the best engineered solution.
uThe TOTEM incorporates decades of automotive expertise in the form of a 1.4 litre Fiat Fire engine sourced from Fiat Chryster Automobiles and an Engine Control Unit tuned by Magneti Marelli.
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Two popular
technologies available are CHP and PV, and with recent changes and the general long‐term uncertainty of incentives, CHP can be the better option.
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enquiries@adveco.co https://adveco.co/ 01252 551 540
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