CPD PROGRAMME Professional development Integrating ChP, condensing boilers and heat pumps in commercial heating systems This module explores some of the factors likely to influence the operational success of integrating different heating technologies 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 longterm 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 CIBSEs requirement for CPD. It will equally assist members of other institutions, who should record CPD activities in accordance with their institutions 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. cPd QUEstIonnaIrE to take part in this months Cpd, simply read through the module here, then follow the link at the bottom of this section. alternatively, visit www.cibsejournal.com/cpd You will receive notification by email of successful completion, which can then be used to validate your Cpd records. Figure 1: Commercial stainless steel condensing gas boiler. yellow line shows gas path to meet combustion air, and then counter-flowing to the system return water that enters (shown as blue) at the base of the boiler, where the flue gases are coolest and where condensation takes place in the combustion gases that are leaving (Source: Elco) Efficiency (nett CV/PCI) % Electrical and heating demand profiles for a simplified fictional building are shown in Figure 4. The diagrams include the output of a possible (and exaggerated) CHP solution both heat and power output of a spark-ignition gas-engine powered CHP that typically produces about twice as much heat energy as electrical energy. In this example, the heating demand fluctuates widely throughout the year, but the electrical demand stays reasonably constant and the summer base load is greater for the heating than the electrical demand. In Option A, the CHP will run continuously, thus ensuring a good payback for the machine. However, some electricity will need to be purchased, and a significant amount of heat is required from associated boilers. A larger unit would offset some of the carbon liabilities and provide a more cost effective solution, as electricity generated at source is more carbon efficient. In Option B, the CHP unit selected is too large. Most of the heat demand is met, so the boilers will be smaller, and some electricity will be supplied back to the grid (sold at only 30-50% of the price of incoming electricity). But if the unit is run all year, there will be excessive heat rejection. The CHP could stand idle during There are many technologies contributing towards meeting the legislative requirements and, more importantly, the environmental imperative of lower carbon heat sources in commercial buildings. This CPD will consider some of the factors that are likely to influence the operational success when considering integrating condensing boilers with combined heat and power (CHP) and heat pumps. the condensing boiler A condensing boiler is equipped with sufficient heat exchange capability to reduce the flue gas temperature to below its dew-point temperature. By doing so, it is possible to recover a proportion of the available latent heat by condensing water vapour from the flue gas, which is a product of combustion. Unless the condensing boiler is installed in such a way as to allow some of the superheated vapour in the flue gas to condense (as in the example shown in is illustrated in Figure 2. This shows the example performance (in net efficiency) of a condensing boiler with modulating output for different pairs of flow and return temperatures. There are many heating systems including individual buildings and district schemes operating at traditional flow and return temperatures of 80C and 60C. Even when a condensing boiler is operating at 80C and 60C return, its extended heat transfer surfaces mean it will still reduce the flue gas temperature (and so gain additional sensible heat), and so will always be more efficient than noncondensing boilers. However, compared with other temperature ranges, in this generalised example there are likely to be real benefits in operating efficiency at lower operating temperatures (as demonstrated in Table 1) and at partload operation. As long as the return temperature is below 55C, at least some of the latent heat that would otherwise be lost will be recovered and, if the boilers are operating at part-load for a large proportion of the time, there is the greatest opportunity for best efficiency. Gas-fired CHP or electrical heat pumps can be used in conjunction 111 110 109 108 107 106 105 104 103 102 101 100 99 98 97 96 95 94 25 35 45 55 65 75 85 95 Efficiency according to DIN 4702 part 6/8 40-20 6 Temperature pair 75/60 50-30 4 Temperature pair 40/30 60-40 3 70-50 2 80-60 1 Figure 2: Comparative performance of modulating output for a condensing boiler at different flow and return temperatures (Source: Elco) the summer, but that is unlikely to be carbon or financially efficient. It could be linked with an absorption or adsorption chiller for summer cooling known as trigeneration although without that facility, it is unlikely to qualify as Good Example load type 40oC 92% Radiators sized for lower temperature 35oC 94% Underfloor heating 30oC 95% Swimming pool water 20oC 97% Pre-heating of domestic hot water Table 1: Systems designed to operate at lower temperatures will enhance boiler efficiency Power station losses 60% (45.0 units) Power station fuel input 75.0 units Boiler fuel input 70.5 units Power station + boiler total primary fuel input 145.5 units Power station 30 units Electricity Energy to buildings Electricity CHP fuel input 116.86 units CHP unit 60 units Typical boiler Heat Heat Boiler losses 15% (10.5 units) CHP losses 23% (26.87 units) Total 90 units CHP total primary fuel input 116.86 units Primary energy saving 145.5 - 116.86 = 28.64 units 28.64/145.5 = 19.68% Figure 3: Simplified comparison of production of heat and electrical energy for use in buildings CHP compared with grid power and local boiler. Note: values used are variable, depending on location and application (Source: Elco) integrated element of the heat source for the main heating system is most effectively done if the recommendations of CIBSE AM12 are followed designing systems with a return temperature of 40C to allow application of renewable technologies. And just as with CHP, heat pumps are most appropriately designed as the lead heat source to maximise running time and potential savings in cost and energy. Heat pump performance will reduce as output temperature rises, as well as when the input temperature falls. For typical electrically powered vapour compression (single stage) heat pumps, the thermodynamics restrict the economic upper limit of the working temperature to below 50-55C. For higher temperatures that are used particularly for domestic hot water and traditional heating systems, a second heat source is required to raise the temperature to 60C and beyond. So, if the heat pump heat source (for example, air or ground) is at a high enough temperature for effective operation, the heat pump will act as the lead-heating appliance and, if required, another heat source such as boilers are brought online to increase the temperature. A heat pump could be integrated in a system as indicated in Figure 5, where option C Load requirement option B Load requirement Load requirement option A Gas-fired combined heat and power Grid-supplied power delivers electricity to end-users with an overall primary energy efficiency of approximately 35% to 40%. A proportion of the loss around 7% is caused by the transmission losses though the electrical supply grid, but the remainder is down to the conversion efficiency at the power station. The simplified example in Figure 3 indicates how localised generation through the use of combined heat and power can potentially provide a more effective use of primary energy. The benefit of CHP is highly dependent on the relative performance of the grid, the available fuel supply and the useful consumption of generated heat and electricity. Buildings that are suitable for CHP schemes are those where there is a consistent demand for heat probably beyond that of a simple 9am-to-5pm office. CIBSE AM122 recommends that for successful CHP integration, the system should be designed to operate at maximum load in preference to boilers for the longest cost-effective period. Heat pumps The inclusion of a heat pump as an Potential condensing boiler gross efficiency Return water temperature with condensing boilers as a means of providing a low-carbon and cost-efficient base load. Quality CHP (as defined by the UKs CHP Quality Assurance Programme3), as heat will be wasted. In this case, Option C provides a better balance. Some heat rejection in the summer may be permissible, as the advantages of meeting the electrical demand outweigh the implications of wasted heat. The installed boiler power is less than Option A. And, naturally, a different set of demand profiles will require specific analysis 30 to 60-minute banded load data is considered appropriate to undertake such an exercise. A concept example of CHP (with potential additional heat pump) integration is shown in Figure 5, as developed from CIBSE AM12. Integrating a thermal store enables the CHP to operate for longer and provides easier control. However, an actual design will still necessarily have reasonably complex control requirements to ensure: minimum flow through boilers; back-end protection; opportunity for CHP excess heat rejection; and, importantly, its most effective operation (the boilers would not be used to charge the store). Capacity (%) Weather compensated 5 Figure 1), thus liberating energy, it will be lost to the atmosphere. The temperature of the water returning to a condensing boiler needs to be less than approximately 55C so that the water vapour will condense cooler temperatures will provide increased latent heat recovery, potentially adding 9% to the useful boiler output. The modulation of the output of a burner is important not only in terms of matching condensing boiler output to load which, in turn, improves seasonal efficiency but also because as burner output decreases, the efficiency of the heat exchanger increases, since the ratio of heat exchange area to input power increases. So at minimum burner input, losses are at their lowest and heat exchanger efficiency at its highest. This phenomenon means that modern condensing boilers are generally more efficient at their minimum output rather than at full output. The combination of beneficial condensation and operating at partial load Electrical power requirement Heating requirement Figure 4: Example of building heat and electrical load profile with different CHP capacities it can act as the lead heat source when effective to do so. This allows good yearround COP on heat pumps, together with suitable return temperatures to the boilers, allowing condensation of flue vapour. Currently, finding both CHP and heat pumps on the same system would be unlikely, as they have similar requirements for success: to operate for long periods, and to act as a base load machine. Capital investment for both is more expensive per kW of delivered heat than with condensing gas boilers. combining the technologies When different heat sources are combined in one system there are considerations of flow dynamics, operational safety and Main flow temperature sensor Temperature sensors to control charging by CHP CHP Boiler 70oC Thermal store hot layer Variable flow distributed load and controls Heat load Boiler cold layer Figure 5: Schematic of example CHP in series with parallel boilers (based on CIBSE AM12, fig 9.4) 50oC Note that this layout provides the principle based on the example given in CIBSE AM12, with the addition of a heat pump in the return system. When applying this in design, it requires practical control measures including: The CHP should lead the boilers in operation The CHP engine will require back-end protection The charging of the thermal store will be from the CHP only The flow into and out from the buffer will require control Refer to section 9.4 of CIBSE AM12, 2014 for further detailed information. temperature control. These are covered in CIBSE AM12 and in greater depth in CIBSE AM15.6 As the UK power distribution grid is decarbonised so improving heat pump viability there may be more opportunity to benefit from an arrangement as shown in concept in Figure 5. The heat pump and CHP unit are not competing for the base load but both are contributing towards it. In low load conditions where the heat pump and CHP need to be supplemented by the boilers, the boilers may still operate in condensing mode, and will likely operate at part load. This system also ensures that the heat pump and CHP will always run to their maximum potential. The success of integrating the three technologies will depend on a proper UK government incentives for assured quality installations CHP can attract Climate Change Levy exemption that can reduce the ROI period by one to two years understanding of the building load profiles and operation across the whole year. This will, in particular, enable the selection of CHP that maintains Good Quality CHP status and so provides benefits financially as well as operationally. Assured quality installations can attract substantial government financial incentives in the UK (see panel below). Return temperatures should be kept as low as possible to maximise the benefits of condensing boilers and improve the effectiveness of heat pumps, and the heating distribution system should be capable of operating on a wide flow/ return temperature differential (at least 25-30K) to reduce energy use, costs and carbon emissions. Tim Dwyer, 2015. 40oC Heat pump and buffer With thanks to Mark Ferris of Elco, who has provided the practical applications used in this article. ReFeRenceS: 1 Conversion factors Energy and carbon conversions 2013 update, www.carbontrust.com/media/18223/ ctl153_conversion_factors.pdf 2 CIBSE AM12 Combined Heat and Power for Buildings, CIBSE 2013. 3 www.gov.uk/combined-heatpower-quality-assurance-programme 4 www.gov.uk/combined-heat-andpower-incentives 5 www.gov.uk/government/ publications/use-of-chpqa-to-obtainenhanced-capital-allowances 6 CIBSE AM15, Biomass heating, CIBSE 2014. Module 77 May 2015 Fill in this months questionnaire online. You will receive notification by email of successful completion, which can then be used to validate your CPD records in accordance with your institutions guidance. Enhanced Capital Allowances for both CHP and heat pumps can save 7-8% of the capital cost over the plant lifetime (and can include auxiliary heating equipment) Renewable heat incentive available for metered heat pump heat Business rating exemption applies to CHP and associated plant CLICk HERE To FILL IN THE quESTIoNNAIRE