CPD PROGRAMME
will operate most frequently. However, if an ‘oversized’ system is used (to satisfy extreme heating equirements), it will spend the majority of operational time at a lower capacity and, therefore, at lower efficiency. In the example, rather than increasing the
system maximum capacity to accommodate infrequent high heating loads, it is likely to be more effective to use a bivalent system, with a boiler that is used when high hot water loads and, typically, low outdoor temperatures coincide.
20% 30% 40% 50% 60% 70% 80% 90% 100% System capacity
Figure 5: Approximate variation in VRV system efficiency at different loadings temperature, and the refrigerant subsequently
flows to the hot water production unit (section 2). The R410a is condensed, exchanging heat with the evaporator of an R134a refrigerant evaporator. Again, using vapour compression, the temperature of the R134a is increased to (potentially) 80°C, feeding the condenser heat exchanger (section 3) to provide hot water. For this example model, the system was
set to supply heat to provide a hot water flow temperature of 60°C. The results of the analysis comparing the
two types of system, as shown in Figure 4, demonstrate some notable differences. Over the year, the VRV/natural gas system provides a VRV coefficient of performance (COP) of 6, combined with a natural gas heating system effectiveness of 0.9, compared to the VRV/ cascade system COP of 6.9. In the cascade system, the modelled VRV yearly output increases to 1,867 MWh (from 906 MWh) because it is providing the heating source for the hot water. With the more advanced heat recovery systems, there are opportunities to readily employ – at very low additional cost – heating or cooling that would otherwise be rejected. At
such times, this can allow the environmental conditions to be more closely controlled to design conditions, at marginal additional cost, potentially improving comfort and occupier satisfaction. The small increase in delivered capacity in Figure 4 is due to the model providing such a benefit. The analysis shows a significant difference between the two systems. The selected system is based on a ‘design’ heating condition. However, in reality, this may be exceeded, so the heating performance of the system should also be analysed at extreme winter conditions. Heat pumps are very efficient at producing domestic hot water, but in the case of a hotel, the comfort of guests is a prime requirement and priority. Further simulations at an extreme winter conditions are likely to indicate that that there could be a shortfall in heating capacity. One solution could be to increase the capacity of the heat pump system, but by increasing the peak capacity of the system it is likely to reduce the year-round efficiency of the system. Figure 5 indicates the general relative performance of a VRV system. The optimum performance is between 50-70% capacity and, typically, this is where a properly-sized system
Integrating AHU cooling and heating coils Traditional methods of cooling and heating the supply air (using separate chillers and boilers, and water-based coils) may be replaced by dedicated VRV system connected coils (carrying ‘refrigerant’) within the air handling unit (AHU). This can eliminate a stage of heat exchange by feeding AHU cooling and heating coils directly with refrigerant, so increasing efficiency. The selection of a VRV system for cooling the supply air relies primarily on the load, but it is important that this load is defined correctly, based on the required on and off coil conditions. A VRV system can also be provide the source
of heat for the AHU heating coil. VRV systems are primarily designed for indoor coils – rather than those in an AHU – and so it is particularly important to check the operational range with the manufacturer to ensure that it can meet the design load. Some manufacturers do not recommend using the coils to heat low temperature air (below 10°C) – this will be dependent on the system supplier. Using VRV equipment as the direct heating/cooling source can provide an efficient application in itself. However, by considering the heating and cooling coils of the AHU systems in conjunction with the coils in the room units can significantly increase the opportunities for heat recovery, as previously waste heat and cooling can be reused elsewhere in the building systems, as shown in Figure 6. With careful analysis and design, VRV
systems can be very effective and efficient at providing air and water temperature control in buildings. As designs become more ambitious in integrating formerly disparate systems, there is a need for more careful analysis and modelling that will rely on closer working relationships between manufacturer, designer and operator to ensure that systems provide seasonal efficiencies that meet expectations. © Tim Dwyer and Richard Green, 2014.
Figure 6: Elements of the building services system recover otherwise wasted heat through the VRV system
www.cibsejournal.com Turn over page to complete module March 2014 CIBSE Journal 67
VRV system efficiency
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