Heat Recovery in Action

08 June 2011

Faced with increasingly stringent carbon reduction legislation, many think ‘zero carbon’ to be an unobtainable Utopia. John Durbin puts the theory into practice and examines the benefits for buildings’ managers.

LAST OCTOBER’S CHANGES to the Building Regulations, F-gas regulations, EPCs and DECs and forthcoming EuP legislation on seasonal efficiencies, all impact upon the installation and maintenance of carbon producing equipment such as heating, cooling and ventilation systems.
However, when integrated into a well designed and fully integrated climate control system, heat recovery solutions can contribute significantly towards carbon reduction and are a major step in the move towards the goal of zero heat rejection.
Every day, many hundreds of tonnes of CO2 are released by waste heat being rejected into the air. It is easy to see that if we capture this waste heat and reuse it where it is needed then we will achieve the highest levels of efficiency.
We need to embrace a ‘whole building thinking’ approach in which heat recovery becomes a vital part of a ‘virtuous circle’ of climate control, rather than taking a piecemeal approach to the individual elements of a system.
The principle is simple. By taking unwanted heat that is generated in areas such as IT suites and reusing it elsewhere in the building, heat can be generated at a much lower cost as well as drastically reducing carbon emissions. However, excess heat can only be used when heat is required. The aim must be to reject heat only after every potential avenue of its re-use has been explored.
The reclamation of heat can be achieved by certain heat pumps that divert heat from indoor units in cooling mode, to areas that require heating. Air source heat pumps can be used in both new build and renovation projects to provide a highly energy efficient solution. They can achieve significant energy cost savings, compared with traditional fossil fuel boiler systems, by producing 65-75 percent of heat from renewable sources heat for buildings and services.
The effectiveness of heat pumps can be maximised by the use of multiple zones on one circuit, so IT and server rooms can be cooled, while other areas and water for washrooms etc can be heated.
Heat pumps are certainly not a new innovation, having been on the market for almost 60 years. However, their history is a tale of continual innovation and the drive for ever greater efficiencies has been relentless. Indeed today’s super-efficient models with inverter control and variable refrigerant flow offer all kinds of new possibilities. The latest advances in super inverter control and integrated heat recovery systems are now capable of delivering previously unimaginable efficiencies.
These innovations have made heat pumps an integral part of many air conditioning, heating, ventilation and refrigeration systems and higher capacity heat pump systems are capable of meeting exacting industrial heating and cooling requirements.
In particular, it is inverter technology that enables truly impressive heat pump performance. This is because inverters adjust the power used to suit the actual requirement.
An inverter unit will gradually increase its capacity based on the need to cool down or heat up the room. While a non-inverter heat pump can be compared with switching a lamp on or off, inverter technology is more like an accelerator in a car, able to adjust continually to the demand required, delivering greater fuel economy.
This provides two clear benefits. An inverter improves comfort levels for building users. Unlike standard on/off systems, an air conditioning system with an inverter continuously modifies its cooling and heating output to suit the temperature in the room. The inverter shortens system start-up time, so that the required room temperature is reached more quickly and temperature fluctuations are avoided. Then as soon as that temperature is reached, the inverter ensures that it is constantly maintained.
Secondly, because an inverter continually monitors and adjusts internal temperature whenever needed, it avoids cycling of the compressor meaning that there are no voltage peaks and energy consumption is therefore 30 percent lower than a traditional on/off system.
The £3.5m refurbishment to No 1 Croydon, formerly known as the NLA Tower, is a good example of heat recovery being incorporated into a refurbishment project. The top nine floors of the famous 1970s octagonal 82 metre high landmark building underwent refurbishment and were fitted with state of the art VRV-WII water-cooled air conditioning. Following a performance specification by consultants, EDCM Consulting, and long term Daikin D1 Partner McDowall Air Conditioning (UK) Ltd, designed, installed and commissioned a VRV-WII water cooled heat recovery system that could fit into the available 250mm ceiling voids and also utilise the existing main central plant.
Replacement of the original perimeter terminal units by the VRV-WII system allowed re-use of both the building’s condenser water circuit and existing air handling plant. All 27 VRV systems, comprising more than 160 fan coil units, were installed and commissioned in 20 weeks, including stripping out the earlier air conditioning units.
Conditioned air is supplied to the open plan office space via slim style Daikin FXDQ ceiling concealed fan coil units located in the false ceiling void and ducted to swirl diffusers mounted, in the main, around the perimeter of the room. Further diffusers are located inboard to allow for future cellular office layout. Return air is extracted through inboard ceiling grilles.
Each floor is provided with three VRV-WII condensing units, mounted singly or double stacked in mini plant rooms adjacent to the toilet areas. Each unit is connected by a two pipe flow and return water circuit and supplied with water via a pressurisation unit at a temperature between 25°C and 35°C depending on the time of the year. Liquid/gas refrigerant lines link the condensing units with the concealed ceiling indoor units installed per floor. Primary tempered and filtered fresh air ventilation is supplied by roof top air handling plant to the false ceiling void and fan coil units where it is cooled or heated.
Fresh air ventilation rate for the offices is eight lit/sec/person based on an occupancy level of one person/12 sq m. Cooling loads are approximately 80kW per floor. Cooling and heating is controlled by two Daikin I-manager air conditioning management systems, utilising the independent Daikin multi-transmission DIIINet high speed communication system and enabling floors to be divided to suit separate tenants.
Another example of heat recovery can be seen at Paramount Office Interiors Ltd in Cardiff. Designed by the company’s own in house design teams, the 14,000 sq ft facility features VRV heat recovery air conditioning. A combination of 26 Daikin ceiling concealed and suspended, four-way blow ceiling cassettes were used with floor mounted cased VRV indoor fan coil units with cooling capacities between 2.2kW and 9.0kW and heating capacities between 2.5kW and 10.0kW.
A dedicated wall mounted split unit handles the communications room requirements. All VRV indoor fan coil units are linked to two 80kW VRV heat recovery outdoor condensing units located in a ground level compound at the rear of the building. Internal design temperatures are maintained at 23°C in summer and 21°C in winter. Overall cooling and heating loads are 153kW and 104kW respectively. Fresh air requirements are supplied via a heat recovery ventilationunit in the plant room.
The heating side alone at full load would only input a maximum of 35 kW of direct heat, the other 65kW coming from renewable aerothermal energy. As the need for heat falls, the percentage of renewable heat provided will rise, thanks to inverter technology. This type of efficiency also applies to the cooling side especially at part load conditions.
In both of these examples, heat recovery air conditioning was chosen to meet the client’s requirement for showpiece installations of all that is modern, stylish and environmentally conscious in office interior design and equipment –and that includes reducing carbon emissions.
● John Durbin, Daikin UK’s Engineering Department Manager