Meeting the climate control needs of commercial buildings
Modern heat pump technology offers an effective means of meeting the climate control needs of many commercial buildings but what are some of the key factors that need to be considered? asks Cliff Jones, head of applied at Daikin.
It is clear that to decarbonise the heating and cooling of buildings, it will require a change in the systems and technologies used, with heat pumps offering major opportunities to lower the environmental impact, especially as the electrical grid itself is decarbonised.
There are broadly two types of heat pump. Air-based systems, also known as air-source heat pumps (ASHP), draw heat directly from ambient air via a heat exchanger. Water-based systems can be divided again into two: Water source heat pumps (WSHP) that harvest heat from surface water, such as lakes, rivers and seawater, and ground source heat pumps (GSHP) that utilise heat from the soil or groundwater.
There is no one-size-fits-all commercial solution and it will often depend on what needs to be achieved. WSHP or GSHP systems deliver more consistent performance throughout the year as ground and water temperatures remain much more stable than air temperatures across the seasons. The UK market is however predominantly ASHP systems as there is not always a suitable water source or the correct geology near the building, meaning WSHP or GSHP are not applicable for many projects.
The requirements of the building are also a factor. Most commercial heat pump systems will be able to fulfil space heating requirements relatively easily, especially for buildings where a lower temperature heating system, such as underfloor heating, is installed. Achieving the higher temperatures of above 70ºC for domestic hot water (DHW) generation has been more challenging for heat pump systems originally designed for HVAC. However, with the move towards lower GWP and natural refrigerants coupled with innovations in heat pump technology there are now ASHP products, such as our own new CZ-HT high temperature heat pump, that can directly achieve domestic hot water of 70ºC across the UK ambient range.
For commercial projects that have comfort cooling requirements, heat pumps can deliver this because the refrigerant cycle can be reversed for cooling. This is one of the key benefits of heat pumps over conventional heating systems.
For buildings with more complex heating and cooling requirements, there are ‘four pipe’ multipurpose heat pump units that can achieve heating, cooling or a mixture of both taking advantage of heat recovery. These multipurpose heat pumps provide an advantage in the intermediate seasons to treat the fresh air load, traditionally covered by gas burners in the central air handling units.
Often a mix of different types of heat pumps will result in the most efficient solution. A combination of ASHP in cascade with a WSHP as a booster to cover the domestic hot water allows the comfort elements of a project to be run at lower, more efficient temperatures and only the required DHW demand to be covered at higher temperatures.
Key design considerations
To ensure that the selected heat pump system can be successfully integrated into a new or existing building, there are several factors that must be considered.
Energy efficiency – Due to the lower flow temperatures and delta-T that heat pumps operate at, it is important to ensure that a ‘fabric first’ approach is taken to maximise the thermal efficiency of the building. The hydraulic design will also need to be evaluated to ensure it adequately covers the different delta-T’s and flow rates.
Power demands – Utilising heat pumps instead of gas boilers is an excellent way to reduce the use of fossil fuels, but they will increase the electrical demand for the building or development, which may have an impact on the electrical infrastructure and connection to the local electrical grid. Therefore, early engagement with the Distribution Network Operator (DNO) is essential to ensure requirements can be met.
Plant size, location and safety – It is important to remember that commercial ASHP plant will occupy a significant amount of space and this needs to be considered at the planning stage, especially where multiple units are required to meet the demand. Cold air discharged from ASHPs has no buoyancy but a natural tendency to dump and adds to the cold recirculation risk in plant areas which needs to be assessed to avoid negative impact on performance and capacity.
Furthermore, all heat pumps generate noise due to the operation of the compressors, fans and valves. Depending on the location, this may need to be addressed to comply with planning requirements. This can be achieved with the use of acoustic enclosures, however these may increase the physical size of the unit and restrict future maintenance, which must also be considered.
For water and ground source heat pumps, which will be installed in an internal plant room, the risk of refrigerant leaks must be assessed and addressed. The manufacturer of the system should be able to provide guidance on refrigerant relief, leak detection and emergency mechanical ventilation measures to mitigate any risks. Also, low GWP and natural refrigerants are classified as mildly flammable (A2L) or flammable (A3). Therefore, it is important to ensure that there is a robust and risk assessed solution in place to limit the hazard to the building.
Defrost derate – As the outdoor coils of ASHPs operate at low temperatures, water vapour in the air will freeze on the coils. This ice needs to be removed to maintain efficiency. To do this, the heat pump’s refrigerant cycle is reversed to deliver heat to the coil. This defrost cycle, which lasts approximately 10 minutes, reduces the heating capacity by around 15% and as a result will impact the water temperatures within the system. There are various control and design solutions to this, including ensuring the system is specified based on the ‘integrated capacity’ – the capacity during defrost – rather than the instantaneous capacity. Also, if the system has multiple units, the defrost cycles should be sequenced so that only one circuit or unit is in defrost mode at a time.
Environmental restrictions – With either WSHPs or GSHPs, the potential impact on the local environment due to the heating or cooling effect of the heat pump’s operation must be considered. This is particularly relevant for open loop systems where water is abstracted from the environment, passed through the heat exchanger and re-injected back into the ground or surface water. It is necessary to obtain required approvals from the Environment Agency.
The right partner – Finally, given the importance of the correct product selection and system design for commercial heat pump systems, it is essential to have the right level of technical support throughout the project. Partnering with an industry leading system supplier who can offer a wide range of solutions to meet every requirement is always recommended. This will help ensure the system operates as efficiently as possible and reliably meets the heating, cooling and hot water requirements of the building occupants.
Commercial heat pumps provide many opportunities for decarbonisation, but it is a technology that has differences to traditional heating designs. It must therefore be understood and applied correctly to ensure success. Daikin has the expertise, experience and solutions to provide the required support at every stage.
