VRF systems that are truly energy efficient
Does the efficiency of MANY VRF systems reflect the efficiency of the units they are made up from? Graham Hendra believes not.
Equipment for building-services systems is steadily getting more efficient. That principle applies to boilers, electric motors, lamps — and VRF air conditioning.
Developments by the major VRF manufacturers over the last 10 years have seen SEERs (seasonal energy-efficiency ratios) increase to such an extent that the energy required for cooling can be reduced by over 60%.
Unfortunately specifying efficient plant items does not always result in an efficient system — a problem that applies just as much to VRF air-conditioning systems as to lighting installations and heating systems.
Graham Hendra, UK general manager of LG’s air-conditioning division, is very much concerned about poor installation practice and lack of design knowledge preventing VRF air-conditioning systems performing to their full potential. He recently aired his views in a seminar at an exhibition. Graham Hendra has a wry sense of humour, which is why he talks about how to minimise the efficiency of your VRF installation through bad design and installation.
A common problem is the positioning of outdoor units, which are often in confined spaces or positioned close to other heat sources, increasing the immediate local temperature and, as a direct consequence, their efficiency.
Graham Hendra explains, ‘Airflow around the outdoor units is a primary concern. Reducing airflow greatly reduces efficiency and, therefore, increases energy consumption. Positioning near other heat sources, such as other outdoor units, can also have a similar effect.’
The EER of an outdoor unit falls off with ambient temperature, with an example quoted by Graham Hendra giving an EER of 6.3 at 20°C and falling almost linearly to 4.5 at 30°C — a 40% increase in the energy required for a particular cooling capacity.
His advice is quite simple: ‘If you can walk round the outdoor unit, it will have enough air.’
Not only has the efficiency of VRF air-conditioning systems improved by leaps and bounds but so too have the permitted lengths of pipe runs. While long pipe runs make possible extensive VRF systems, the capacity of indoors units sited a long way from the outdoor units falls off substantially. Graham Hendra tells us that manufacturers quote the cooling capacity of indoor units for a 7.5 m pipe run. At 50 m, the capacity falls off by about 8% and by up to 15% at 100 m. By 150 m, the fall-off in capacity could be 20%, so a 7 kW cassette would deliver only 5.4 kW of cooling. The loss in capacity is due to the coil of the indoor unit operating at a higher temperature, so that the difference between its temperature and the air in the space is smaller
There are several ways to regain this lost capacity, advises Graham Hendra.
One is to make the outdoor units work harder — which uses more energy.
Another is to increase the size of the pipework if the distance to the furthest indoor unit exceeds 90 m — although he has seen this happen only twice in 10 years and with experience of 600 installations.
The third, and best, approach is to keep pipe runs short.
One of the features of many VRF systems is being able to cool some parts of a building while heating others. While that looks like an attractive feature, it significantly increases the capital cost — and the payback time can be very long indeed.
Graham Hendra explains why by considering a system that spends a third of its time cooling, a third heating and a third in heat recovery. He then suggests that a 30 kW VRF system in an office of 250 m2 would cost £1500 a year to run — £500 each for cooling, heating and heat recovery. Assuming that heat recovery is used all the time, the reduction in running cost would be just £250 a year.
Basing his analysis on the installed cost of a 2-pipe VRF system being £125/m2 and for a 3-pipe heat-recovery system £155/m2, the extra cost for the heat-recovery capability would be £7000 — representing a 28-year payback!
If a heat-recovery VRF system is being installed, it is important to provide the opportunity for heat recovery. Graham Hendra has seen separate systems on the north and south aspects of a building, so there is nowhere for recovered heat to go. A better approach is to install systems floor-by-floor so that a cooling load on one side of the building can be balanced by a heating load on the other.
A different perspective on heat recovery is provided by water-cooled systems, with heat transferred between refrigerant pipework and the water loop by units around the building.
Another feature of VRF systems that often concerns designers is the total cooling capacity of indoor units exceeding that of the outdoor units — known as the connection index. Graham Hendra explains that the concern is that VRF systems with more than 100% connection ratios will not be able to serve all indoor units if they are all running flat out. That concern is not true in cooling operation, although it has some impact in heating operation.
What happens in cooling operation is that as the load rises, the temperature of the heat exchanger rises, making it less efficient and reducing the EER. He says, ‘If you want pure energy efficiency, keep the connection ratio low. But you also have to ask how often all the capacity will be required. Installing larger outdoor units is efficient, but initially more costly.’
Heating presents a slightly different picture, and there is a small drop in capacity with a connection ratio of over 100%. However, as Graham Hendra points out, VRF systems have a higher heating capacity than cooling capacity, it rarely has a noticeable effect in real buildings.
As with any building-services system, a VRF installation will be inefficient if it is not properly controlled — and it is controls that make possible enormous energy savings.
Users like to adjust temperatures, so Graham Hendra suggests limiting the maximum and minimum temperatures. Likewise, time clocks can be set with a final off time to prevent units being run all night if they can be operated locally.
If a building is cooled by a VRF system but heated by a wet system, their controls should be interlinked to prevent one system fighting the other.
Energy will be wasted by ventilation systems if they are allowed to run when an area is not occupied — and they should also be linked to the VRF controls.
Graham Hendra says that the widely available software for designing VRF systems only provides information about the capacity of indoor units once the system has been designed, but that it does not tell you how to increase efficiency or capacity.
In summary, there are four key issues to consider to maximise the efficiency of a VRF air-conditioning system.
• Keep field piping as short as possible.
• Keep the diversity percentage down.
• Let the outdoor units breath.
• Make full use of the sophisticated controls available — and advise how best to set up and use them on site.
