Guy Cashmore, Technical Director for UK manufacturer Kensa Heat Pumps, clarifies what is meant by the term ‘cascaded ground source heat pumps’; and what this type of system architecture can bring to commercial projects with high heat loads.
What is the difference between cascaded boiler systems and ground source heat pump systems?
Most commercial heating and plumbing contractors will be familiar with cascaded boiler systems. They are generally used in projects with large heat loads, and/or critical applications where a degree of redundancy is required within the heating system.
With a cascaded boiler system, the usual route is to set up the boilers to operate at the temperature needed by the circuit requiring the highest temperature. Mixing or blending valves are then used to reduce the temperature down to what is needed by other circuits. Boilers are often set up to run at temperatures well above the maximum required and as the efficiency of the system is virtually unaffected this is a perfectly logical approach.
If this route was taken with ground source heat pumps however, although the system would function, in most cases it would be inefficient and result in significantly higher running costs for the client; Kensa would not recommend this approach! In order to enjoy the many benefits of ground source heat pumps and efficiently deliver the heating and hot water demands of commercial projects, there is a far more elegant and efficient solution…
What is a cascaded ground source heat pump system?
Refrigeration engineers might understand a cascaded ground source heat pump to mean a unit containing multi-stage compressors designed to take low grade heat energy and convert this to a much higher temperature. A cascaded ground source heat pump system, however, can be defined in various ways.
Cascaded ground source heat pump systems feature a central ground array infrastructure, made up of either slinkies, or boreholes, that is sized to deal with the entire heat demand (peak and annual). Typically a cascaded system will link this ground array with a number of large modular ground source heat pumps (like the Kensa Twin Compact or Kensa Commercial Plant Room models) together all in one central plant room. This system configuration enables the heat and/or hot water to be delivered by individual dedicated cascaded systems, within which the heat pump units can either be working together to meet peak heat demand, or one unit can operate in isolation when demand is low.
Projects utilising this type of cascaded ground source heat pump system design are eligible for the Government’s Non-Domestic Renewable Heat Incentive (RHI), which provides the system owner with a guaranteed quarterly income for 20 years based on the actual heat output of the system.
Why use a cascaded ground source heat pump system architecture?
Cascaded ground source heat pump systems can adjust between minimum and maximum heat demand as needed, which means that the system can take into account seasonal demand differences and optimally produce the required amount of heating.
For example, in winter a project might need 100kW of heat to satisfy demand, however during the spring and summer the need for heat may be more like 40kW. In order to avoid inefficiencies, a cascaded ground source heat pump system comprising of two 50kW units might be installed. When a load greater than 50kW is needed, then both heat pumps will operate to fulfil demand, whilst only one need operate in the summer and shoulder seasons.
If the property requires heating and hot water, cascaded systems can also be designed with one or more heat pumps dedicated to the production of domestic hot water only, and the remaining units dedicated to satisfy the space heating load. Specialist high temperature ground source heat pumps can be used to produce the domestic hot water, removing the reliance on immersion heaters.
One example of this is at Hornsea Garden Centre near Hull. The owners were looking to heat a new extension and upgrade the main building to underfloor heating. Kensa specified a cascaded ground source heat pump system featuring two 45kW Plantroom units to cover the heating, and one 25kW High Temperature Plantroom unit to provide hot water.
Are there any other benefits to cascaded ground source heat pump systems?
Using multiple ground source heat pumps in a cascaded configuration has other advantages, too. Ground source technology gives the best efficiency when operated with long run cycles. Modulating ground source heat pumps, either by frequent stop-start or by using inverters, invariably causes efficiency loss. Having a cascade of ground source heat pumps means that the majority of the load will be supplied by units running almost continuously.
It also means that in the unlikely event of a heat pump breakdown, only a small % of the total capacity is lost, so the building isn’t going to go completely cold whatever the weather. It also means that engineers can work on each individual heat pump separately, whilst the rest of the system remains operational.
Significantly, as ground source heat pumps operate at low flow temperatures, this not only improves overall efficiencies but reduces overheating, which is particularity common in risers and corridors in the summer. This is most noticeable on older systems where the pipe insulation is not up to modern standards, and improving it is a logistical challenge.
How can you get the best efficiencies?
Keeping flow temperatures only as high as necessary reduces heat losses. The efficiency of ground source heat pumps is closely linked to their output temperature, as ground source heat pumps are low flow temperature devices. The lower the flow temperature required from the system, the higher the efficiency of the heat pump and the lower the running costs will be.
The efficiency of a traditional boiler (the conversion ratio between fuel and heat), is not greatly affected by the output temperature. Running the boiler at 40°C compared to an output of 60°C will only change the efficiency by a few %. However with a ground source heat pump nothing could be further from the truth, as the efficiency difference at the same temperatures could be a change from 375% down to 225%!
Great care therefore needs to be taken to accurately model the maximum required temperature, and all sensible measures taken to reduce this maximum temperature, e.g. increasing the size of the heat emitters or increasing insulation levels.
How do you deal with differing temperature requirements?
Many commercial buildings have multiple temperature requirements across different heating distribution systems. A recent example encountered by Kensa needed three different temperatures; 50°C for the old radiator system (this may sound low, but after the building was insulated and had the single glazed windows replaced, a re-calculation of the existing radiator sizes showed this was ample); 35°C for the underfloor heating in the new extension; and 65°C for the domestic hot water system.
In this office building the radiator load was small, around 30% of the total heat requirement. It would have made no sense at all to run all the heating at 50°C and ignore the higher efficiencies possible on the underfloor heating circuit. In this case, retaining the common ground array but installing separate ground source heat pumps optimised for each circuit, was the obvious way forward.
The key difference between this approach and the typical cascaded ground source heat pump installation described previously is that on the (hot) output side the heat pumps won’t all be feeding into a common distribution system, the heat pumps will instead be feeding two or three dedicated circuits with differing temperature requirements. Of course it is possible that should the need arise multiple heat pumps can be cascaded should there be a big demand for heat on one or more circuits, thus effectively forming a series of cascaded systems in one system.
In summary, a cascaded ground source heat pump system architecture can be effectively used to meet a high heat load, adjust to seasonal variations, maximise efficiencies of multiple temperature zones, add redundancy, and reduce overheating.