CPD PROGRAMME Professional development Integrating centralised hybrid heat pumps with independent room units for energyefficient concurrent heating and cooling The CIBSE Journal CPD Programme This module looks at how independent room units are successfully integrated with centralised hybrid heat pumps for energy efficient heating, cooling and hot water 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. water circuit can be used either for space heating or for cooling, but not both at the same time. A summer/ winter switch is used for a seasonal changeover so simultaneous cooling and heating in the same system is not possible. Successful operation of changeover two-pipe systems in climates such as that of the UK is likely to be challenging and rarely applied. Both four-pipe and two-pipe systems are typically served by either a chiller and boiler combination or a heat pump arrangement. In commercial or institutional buildings, cooling and heating loads often coincide. Traditional systems with separate chillers and boilers do not allow the recovery or shifting of heat from one space to another. A hybrid heat pump allows this energy potential to be recovered and usefully applied. This CPD will consider how modern variants of water-loop room systems are successfully integrated with centralised hybrid heat pumps to provide year-round energy-efficient heating, cooling and hot water, with reduced lifecycle cost and environmental emissions. There are numerous global applications of four-pipe and twopipe water distribution systems serving room terminal units most typically, fan coil units (FCUs) comprising a filter, coil(s), possibly condensate drainage and a fan that provide heating and cooling to rooms. In CIBSE TM43, a simple analysis of the CO2 emissions for a fan coil unit system installed in an office-type example building compare favourably with most other HVAC systems. If used with a well-designed fresh-air air-handling unit (AHU), a fan coil unit system will comply with Building Regulations, and is surpassed only by more expensive and less flexible systems, such as chilled beams/ceilings. Four-pipe systems have two independent water circuits one with chilled water for room cooling, the other with hot water for heating. All terminal units in four-pipe systems are equipped with two independent coils, and can cool or heat according to space requirements. Four-pipe systems are extensively used in temperate climates such as the UKs, particularly where there is no clearly defined seasonal operation. No summer/winter changeover is required, as cooling or heating can be produced at all times, and the control of the temperature of each room is independent of others. In comparison, a single, two-pipe, Hybrid heat pump technology A hybrid heat pump is a packaged heat pump equipped with a flexible and versatile heat-recovery system, which offers the options to deliver cooling only, heating only, or cooling and heating at the same time. Each unit is equipped with three heat exchangers: the so-called main heat exchanger, where chilled water is produced; the heat recovery or secondary heat exchanger, where only hot water can be produced; and the condenser/evaporator, where heat rejection or heat absorption takes place. This last heat exchanger can be a finned coil, in the case of aircooled units, or a refrigerant-towater heat exchanger, in the case of a water-cooled unit. In each operating mode, only two heat exchangers are activated. When only chilled water is required, the unit will operate like a normal chiller the heat will be removed from the main heat exchanger and rejected at the condenser (A1 mode in Figure 1). When chilled water and hot water are required at the same time, the unit will switch to heat-recovery mode the heat removed at the main heat exchanger producing chilled water will be rejected to heat recovery, producing hot water (A2 mode in Figure 1). If the chilled water requirements are satisfied, but there is still a demand for hot water, the unit will switch to heat-pump mode, using the third heat exchanger as the evaporator and rejecting the heat to Only cold water production in the main exchanger (A1) Cold water production in the main exchanger and hot water production in the secondary exchanger (A2) (recovery unit) Only hot water production in the secondary exchanger (A3) (recovery unit) Figure 1: Working principle of the single refrigerant circuit in a hybrid heat pump (E = main heat exchanger; C = compressor; V = expansion valve; R = secondary heat exchanger; S = condenser/ evaporator)3 Figure 2: Principal arrangement of a four-pipe system connected to a hybrid heat pump. The main heat exchanger is providing chilled water to the space cooling circuit, and the secondary heat exchanger is supplying hot water to the space heating circuit3 Space heating circuit Expansion vessel Make-up boiler Hot water tank AHUs hot coils Expansion vessel Secondary Main Space cooling circuit Hybrid heat pump Expansion vessel Chilled water tank the main heat exchanger, or to heat recovery, producing hot water (A3 mode in Figure 1). The unit can change its operating mode, according to system requirements. If the unit is equipped with two independent refrigerant circuits, each circuit can operate in A1, A2 or A3 mode independently from each other. The control logic of the unit will optimise the operation of each circuit to minimise the energy consumption. Traditional systems, including chillers or traditional heat pumps have a typical energy efficiency ratio (EER ratio of the cooling output to the total power input) of approximately 3 (air-cooled) and 5.2 (water-cooled), or heat pump coefficient of performance (COP ratio of the heating output to the total power input) of approximately 3.2 (air-sourced) and 4.5 (watersourced). Hybrid heat pumps can deliver typical values for a total efficiency ratio (TER ratio of sum of cooling plus heating outputs to total power input) in the range of 7 to 9. The secondary heat exchanger of the hybrid heat pump can be connected to the domestic hot water primary circuit. Air-sourced units with semi-hermetic R134a screw compressors can operate with outdoor air temperature down to -10C while maintaining economic hot water production up to 50C. In a four-pipe system (Figure 2), the main heat exchanger only provides chilled water to the circuit dedicated to space cooling, while the secondary heat exchanger supplies hot water to the circuit dedicated to space heating. Air-sourced R410A units with scroll compressors have been designed to produce hot water at 45C during normal operation with an outdoor air temperature of -10C in winter. A make-up boiler is indicated in Figure 2, as it may be necessary, in some cases, to top up the heating capacity, or the hot water temperature. Meeting the heating loads Traditional flow water temperatures of 80C are not available from heat pump technology, so the fan coil units must be designed to provide the appropriate heat output at flow water temperatures closer to 45C. Example application In Milan, a 40-year-old, four-storey office building was refurbished and extended to create a further two floors. The project replaced the existing two-pipe heating and cooling plant comprising a liquid chiller and a boiler with a hybrid heat pump capable of producing both chilled water and hot water, independently and simultaneously. The L-shaped building had different exposures, so areas had very different heating and cooling loads, with concurrent opposite loads, particularly in mid seasons. Cooling was required from external temperatures of -5C (15% max load) to 35C (100%), heating from -5C Figure 3: Hybrid R134a screw compressor heat pump 550kW cooling capacity with 7C chilled water at 35C ambient, 396kW heating capacity with 45C hot water at -5C ambient external (100%) to 18C (20%). The old two-pipe system was replaced by a more versatile four-pipe system that supplied 263 FCUs and two AHUs from a hybrid heat pump (Figure 3) with a nominal cooling/ heating capacity of 550kW/396kW. The actual measured performance of the system was compared with a model of a traditional system that used a liquid chiller for cooling and a 35 30 Milan Max OC 25 Milan Min OC 20 London Max OC 15 London Min OC 10 Stuttgart Max OC 5 Stuttgart Min OC 0 -5 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec -10 Figure 4: Average minimum and maximum external temperatures for Milan, Stuttgart and London (based on monthly data) (Data source: www.worldweatheronline.com) cooling and heat recovery mode (A2 operating mode in Figure 1) and the other circuit operating in heating only mode (A3 operating mode in Figure 1). Because of the Milan climate, no additional boiler was needed for winter top-up, so no gas connection was required for the building. From 35C to 18C external temperature, the unit operates as a chiller (A1); from 18C to 6C, it Primary energy consumption (kWh/year) 1,400,000 1,200,000 1,000,000 -29% 800,000 600,000 400,000 200,000 0 Traditional system Milan Hybrid heat pump system Stuttgart CO2 emissions (kg) 250,000 200,000 -28% 150,000 operates in cooling and heat recovery mode (A2); from 18C to -5C, it provides cooling/heat recovery and acts as a heat pump (A2 and A3). By undertaking a 15-year lifecycle cost (LCC) analysis of the two solutions, the capital cost of the hybrid heat pump solution is 28% higher, but it offers a payback period of 1.2 years, while the lifecycle cost is 23% lower. are more hours where simultaneous heating and cooling are required, and where the heat recovery mode (A2) can be applied. The heating energy available as free heating reduces the amount of primary energy needed to satisfy the heating loads of the building, thus resulting in the potential reduction of CO2 emissions, as shown in Figure 5. Conclusions The use of a hybrid heat pump can provide a significant improvement to the environmental performance of a building with disparate loads, by reducing running costs and lifecycle costs, plus primary energy use, and so cutting environmental emissions. With appropriate systems, this can be condensing boiler for heating. The quantity of heat available from the secondary heat exchanger is enough to provide the heating requirements of the building from external temperatures of 18C down to around 6C. For lower outdoor temperatures, the heating and cooling loads of the building will be covered by the hybrid heat pump operating one refrigerant circuit in A similar system comparison was performed by modelling the same building in the more extreme climatic conditions of Stuttgart, Germany. (For a comparison of outdoor air temperatures, see Figure 4.) Stuttgart has a significant number of hours with external temperatures lower than -5C, even during daytime, with a recent minimum recorded temperature of -12C. In these conditions, all-year-round use of a heat pump is not possible, and a condensing boiler was added to the model for operation when outdoor temperatures were below -5C. The modelled behaviour of the system is similar to that of the real case in Milan, with the principal difference being that when the free heating is not available to cover the heating load of the building, the hybrid heat pump will operate as a heat pump (A3 operating mode) down to -5C outdoor temperature. The capital cost of the new Stuttgart system is 54% more expensive than the traditional system, but provides a payback period of significantly less than two years, and LCC will be reduced by 25%, assuming 15-year operation. In Stuttgart, there successfully implemented across a wide range of climatic zones. TimDwyer,2015. The examples and core explanation of the hybrid heat pump system in this CPD were based on work undertaken by Janes, M et al of Rhoss SpA. REFERENCES: 1 CIBSE TM43 Fan coil units, 2008. 2 CIBSE TM53 Refurbishment of nondomestic building, 2013. 3 Janes, M et al, Energy efficiency in the modern buildings: Energy saving through the application of hybrid heat pumps with simultaneous and opposite loads a case history and a numerical simulation for a four-pipes system, Rhoss SpA, Italy, 2014. Module 83 October 2015 100,000 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. 50,000 0 Traditional system Milan Figure 5: Comparative CO2 emissions and energy use3 Hybrid heat pump system Stuttgart CLICK HERE TO FILL IN THE QUESTIONNAIRE