Extending the energy-efficiency capabilities of VRF systems
The heat-recovery and heat-pump capabilities of a VRF system can be exploited to deliver more than air conditioning — with substantial energy-efficiency benefits. Andy Slater of LG explains how.
With a rapidly increasing market for VRF heat-recovery technology, the ability to increase operational efficiencies is becoming more of a realisation than simply a design potential. In its basic form, a typical VRF heat-pump system can remove energy from one space and reject it into another, for example when cooling. The energy would be removed from the indoor space and rejected outside, with all indoor units of only having the option of operating in the same mode — whether that be cooling or heating.
In contrast, a heat-recovery VRF system provides the capability of connected indoor units operating in different modes — heating or cooling. Energy extracted from a room being cooled can be used to heat other rooms, thereby achieving an energy balance. There is the potential to reduce energy consumption by 30% and achieve a COP of up to 8.5.
Whilst traditional heat-recovery operation provides energy transfer between fan-coil units, it is also possible to transfer heat to water via equipment such as LG’s Hydrokits. With a system-connection capability matching fan coils, Hydrokits can achieve a leaving water temperature of up to 80°C, collectively have a potential heating capacity of 30 kW from a single unit, as well as producing cold water production down to 6°C.
Whilst it is possible to operate Hydrokits as the sole type of unit connected to a VRF or as the only unit operating on a system, the real system efficiency gain is being operational when other connected items are operating in an opposite mode. It is now common practice for a commercial VRF specification to include some method of Hydrokit application, whether to provide domestic hot water or hydronic heat exchangers inside ductwork.
Care should be taken when designing VRF projects with Hydrokits to maximise efficiency gains, taking into consideration water flow rates, water temperatures and the method of emitters being used. By understanding a Hydrokit’s potential and its operational values, it is easier to plan the integration with other building services. For example, whilst most people utilise a low-loss header or thermal store with a supply of water at a single temperature to provide multiple emitters requiring different temperatures, a variation in leaving water temperature from a Hydrokit will have an effect on how efficiently it is performing.
Most installations are set to a maximum leaving water temperature or a temperature to suit the maximum requirement of the installation. However, more and more situations are now showing that water provided to other parts of an installation, for example underfloor heating, is being blended to a suitably lower temperature.
This approach causes two potential losses of efficiency. One is an initial loss in efficiency by producing the unnecessarily high water temperature for that application; the other is the increased difference in temperatures between flow and return at the Hydrokit.
To provide assistance to this type of application the Hydrokit has the potential to provide two flow/return water temperatures and output signals to valves to divert the flow. For example, there can be a lower setpoint for underfloor heating and a higher setpoint for potable hot water, maximising efficiency gains and serving each area as required.
A self-regulating weather compensation setting will also add value to efficiency gains. Monitoring the outdoor ambient temperature, the Hydrokit can regulate its water temperature between points selected at commissioning, enabling heat emitters to avoid overshooting temperature in a design space.
In most commercial projects that include provision for both space heating and potable hot water, the Hydrokit is selected on capacity and maximum leaving water temperature for the entire load, so efficiency is not maximised.
In a basic example, when the total load is analysed, it is usually found that about 80% is for space heating and about 20% is for potable hot water. In most cases the designation of 100 kW of high-temperature Hydrokits to reach the temperature required for domestic hot water is unnecessary and increases the capital cost for the project and reduces the potential operating efficiency.
If the during the design stages an understanding of the overall requirements is taken into account, 80 kW of mid-temperature Hydrokits could be installed to deliver a water temperature suitable for underfloor design, with a further 20 kW of high-temperature Hydrokits to further raise the water temperature for storage in a calorifier.
Most of the load would be met using R410A to heat water, with the extra temperature required for domestic hot water being achieved using cascade R134a transfer.
Whilst the development and availability of new technologies is paramount to maximising energy saving methods, it is also important to fully understand their full potential in terms of application and specific inclusion at the initial design stages in order to deliver consistently high efficiencies.
Andy Slater is technical research and development manager with LG.