Cascade Ground Source Heat Pump Systems

What is a cascaded system for heat pumps?

A cascaded ground source heat pump system allows heat pump units to work together to adjust to meet a property’s peak heat demand. It can be used to meet a high heat load, adjust to seasonal variations, maximise efficiencies of multiple temperature zones, add redundancy, and reduce overheating. When the demand for heat is low in the summertime, a unit in the system can operate in isolation to optimise efficiency and reduce running costs.

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, typically made up of either slinkies or boreholes. This infrastructure is sized to deal with a building’s entire peak and annual heat demand. Typically a cascaded system will see the ground arrays connected to numerous large ground source heat pumps. These are then linked together in one central plant room.

What properties are cascaded systems ideal for?

Cascaded heat pump systems are particularly useful in commercial applications. Not only do they efficiently meet fluctuating heat demands, but each unit can have its own use. For example, one could be dedicated to providing heating while another is providing hot water. This avoids any delay in demand, making the set-up ideal for businesses.

How do cascaded ground source heat pump systems work?

Cascaded ground source heat pump systems are designed to adjust between minimum and maximum heat demand as needed. This means that the system is highly efficient, as it only produces the necessary amount of heat and hot water.

Cascaded systems can be designed to satisfy space heating and domestic hot water demands at the same time. These types of systems feature multiple ground source heat pumps with one or more dedicated to the production of hot water only and the remaining units dedicated to the space heating load. The advantage of splitting the roles of the heat pumps is specialist high temperature ground source heat pumps can be used to produce the domestic hot water. This removes any reliance on immersion heaters. These types of cascaded systems may consist of three, four or even more heat pumps, where the domestic hot water and space heating loads are split again.

Why use a cascaded ground source heat pump system?

Efficient system for high heat demands

Cascaded ground source heat pump systems can adjust between minimum and maximum heat demands. This means that the system takes seasonal demand differences into account and economically produces heating without waste.

Ground source technology gives the best efficiency when operated with long-run cycles. Modulating ground source heat pumps, either by frequent stop-start or the use of inverters, invariably causes efficiency loss. Having a cascade of ground source heat pumps means units can run almost continuously – for example, one can be dedicated to delivering hot water while another delivers the heating.

Reduced risk of downtime

In the unlikely event that one unit breaks down, the system will continue to operate. Although the full heat load cannot be met, a degree of heat will be maintained in the building until the faulty unit is repaired or replaced or repaired. Engineers can then work on an individual heat pump while the rest of the system remains operational.

Prevents overheating

As ground source heat pumps operate at low flow temperatures, this not only improves overall efficiencies but reduces overheating. Overheating is a common problem in risers and corridors in the summer. This is most noticeable in older systems, where the pipe insulation is not up to modern standards and improving it is a logistical challenge.

How do you get the best efficiency from a cascaded heat pump system?

The efficiency of ground source heat pumps is closely linked to their output temperature, as they are ideally low flow temperature systems. The lower the flow temperature of the radiators or underfloor heating 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 percent. However, with a ground source heat pump, the efficiency difference at the same temperatures could result in a drop from 3.75 CoP to 2.25 CoP.

To accurately model the maximum required temperature of a heat pump, sensible measures can be taken to reduce it. For example, a building could increase the size of the heat emitters – such as radiators – or increase insulation levels.