whereas other applications can provide efficient hot water production and, concurrently, provide the space-conditioning requirement by utilising heat recovery technology.
Modelling advanced VRV application to supply ‘high temperature’ domestic hot water for an example hotel A good potential application of using a VRV-linked system to provide domestic hot water with high efficiency is in hotels. A thermal model was created to explore a hotel’s cooling and domestic hot water requirement to establish what could be reasonably, and effectively, produced by a VRV system. The hotel was based on standard 2002 regulation UK constructions, with facilities on the ground floor for a lobby area, gym, meeting rooms, office, restaurant and kitchen with the four floors of guest rooms above. For this modelling exercise, it was located in Strasbourg, France, a region that has a climate slightly more extreme (both in summer and winter) than London. The building was analysed using IES
VE software in conjunction with a ‘plug-in’ designed specifically to model the operation of Daikin VRV systems. The model had each space analysed in detail to identify the best possible performance for a heat recovery system. This was a lengthy process, and was undertaken to evaluate the opportunities for the technology, going beyond the requirements of a ‘normal’ design process. (This in-depth analysis is not yet available commercially.) The overall (seasonal) performance of a
VRV-based system is operationally determined by the heat removed (as cooling) from the building, combined with that produced by the compression cycle being matched by a heating demand. The building’s north- facing guest rooms and the lobby area (with constantly opening and closing doors) provided a heat demand, particularly in the mornings, although the majority of the building’s air load was for cooling – some areas having significant heat gains, such as the restaurant, meeting rooms and south-facing guest
rooms.The internal space and fabric loads were combined with the domestic hot water requirements to provide an annual heating energy requirement of 1,233 MWh and an annual cooling energy requirement of 922 MWh. This indicated that there was opportunity
to transfer available heat usefully around the building. However, the key – and more challenging – task was to provide a realistic and detailed simulation of building operation so that simultaneous heating and cooling loads could be matched to investigate the potential for heat recovery. Heating the domestic hot water can provide an efficient means of
66 CIBSE Journal March 2014
400 350 300 250 200 150 100 50 0
Mon Tue Wed Thu Fri
Heating load Cooling load
Sat
Figure 2: Hotel heating and cooling demands over a week in April Date: Monday 5 April to Sunday 11 April
matching the cooling for very little additional energy. For example, in Figure 2, for a particular
week in April there is on average around 80 kW of continuous free capacity available and, at some points, as much as 220 kW of capacity that can be recycled for use elsewhere in the building. A ‘traditional’ VRV-based system might well
operate with a heat pump VRV system for air temperature control, and a natural gas supply providing the heat required to satisfy the hot water demand. A development of this system would be to
use a (low temperature) VRV hot water system that uses the heat rejected from the building and compressor to raise the domestic water
Outdoor unit AIR FLOW
Ambient down to -200
C C Heat exchanger Expansion valve C Compressor Figure 3: Cascade refrigeration to effectively produce hot water from heat rejected by VRV system Heat pump plus
natural gas hot water production (@60°C)
Delivered capacity
Gas use for hot water heating Electricity to power VRV Annual cost
Tonnes CO2 e per kWh
1,843 MWh 1,101 MWh
151 MWh (@ COP=6)
£62,208 270
Calculations based on electricity at £0.12 per kWh, 0.430 kg CO2 kg CO2
Heat pump with HT hot water
production (@60°C) 1,867 MWh
No gas used 272 MWh
(@ COP=6.9) £38,544 120
per kWh, and natural gas at £0.04 per kWh, 0.185 Figure 4: Modelled annual carbon emissions and energy costs
www.cibsejournal.com C R-410A R-134A 1
temperature. Practically, this can raise the water up to a maximum of 55°C, and so this system typically requires additional gas or electric ‘top-up’ heating to maintain safe temperatures for Legionella control. That type of system was not considered in the comparison modelled in this example. The more advanced application is a
VRV heat recovery system utilising high temperature domestic hot water production via a ‘cascade’ hot water exchanger. A cascade system uses two vapour compression cycles as shown in Figure 3. A R410a refrigerant circuit has an evaporator in the outdoor unit of the VRV system (section 1), drawing heat from that rejected by the VRV cycle. The compressor increases pressure and
2 3 Indoor unit
TO HEATING SYSTEM
Flow up to +800
C
Sun
Mon
System load (KW)
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