The word “cascade” can mean a couple of different arrangements of heat pumps.
1) A series of heat pumps joined together to produce a great amount of heat. E.g. 4x 50KW for 200KW. This arrangement is also known as multiplex.
2) A series of heat pumps stacked together where one supplies the next etc. in order to generate specific temperatures. There are normally two but for some applications, e.g. really high temperature, it could be three.
Multiplex Cascade Heat Pumps
Multiplex cascade heat pumps work by having multiple individual units joined at a single control panel. Through this control system the heat and hot water needs of the property can be managed, transferred and split across the heat pumps to maximise efficiency and performance. Depending on the conditions, this may mean that they are working together on the same task (such as heating the building in very cold weather) or split, with one providing heating and the other providing hot water.
Use-cases For Multiplex Cascade Heat Pumps
For commercial properties, cascade heat pumps can be very used if you don’t have modulating heat pumps and have exceptionally high and/or fluctuating demand for heat and hot water or a need for certain levels of redundancy in the system. Many commercial properties with these kind of demands will already have cascade boiler systems – and for them a cascade heat pump system could be an ideal route to decarbonisation. However Clade heat pumps are fully modulating, in which case it is not necessary to cascade which is often cheaper too.
Multiplex cascade heat pumps can be good for resilience and are often used with non modulating heat pumps to give an overall modulating output. With Clade heat pumps, which are fully modulating, it is not necessary to do this. Resilience is a good reason to multiplex but we do not recommend more than 2 and sometimes 3 heat pumps because the costs outweigh the benefits.
Cascade for temperature
For some installations a straight boiler replacement is required at normal boiler temperatures of around 80/70C flow and return.
Heat pumps are amazing and can achieve this but require two stages to do so. Clade use two hydrocarbon refrigerants, r290 and r600a in an ASHP and WSHP respectively. These are in fact versions of our Aspen and Willow heat pumps with buffer tanks between to make operation as efficient as it can be.
This is an example installation to demonstrate the characteristics of a cascade heat pump system. In this instance the system is designed to deliver the full duty of 560KW at 80/70⁰C. In other systems the cascade may only supply part of the duty.
The advantage of this arrangement is the direct replacement of boiler technology without affecting the wider heating system.
The disadvantage is the increased electrical load (two sets of compressors) which will require a larger supply and the space taken. The complexity of the system can also increase the operational and maintenance costs.
The system consists of two ASHP and two WSHPs sized to match each other. Intermediate buffers smooth the operation by disconnecting the two heat pumps.
-2 x 280kw r600a high temperature WSHP with a COP of 3.89
-2 x 160kw r290 low temperature ASHP with COP of 2.81
-System COP of 2.1 (=280/(72+57)
The major advantage of a cascade or two stage heat pump is that is works at boiler temperatures of 82/71 or similar. The low temperature air source heat pump supplies heat to the high temperature heat pump which elevates the temperature. The two have to be carefully matched and buffers between them help smooth operations.
The familiar temperatures generated make for easy retro fit, no emitter changes are necessary. Controls can stay the same and so the work inside the building is minimal. On the face of it this can appear to be the lowest cost and least disruptive option to install.
The disadvantage to a cascade system is the running cost. Essentially two sets of compressors are being run to produce the one output of heat. This means the overall system COP (coefficient of performance) is significantly less than a single stage heat pump.
A cascade heat pump is a great example of the flexibility of the technology to achieve all manner of temperatures and service. But, the full life cost should always be considered and the end user must understand the implications.
Do Clade Provide Cascade Heat Pump Systems?
We do – in the form of our Larch Cascade Heat Pump System. It is a combination of our r290 air source heat pump and r600a water source heat pump.
You can download a datasheet here or contact us for more information.