What Is A Cascade Heat Pump System?
A cascade heat pump system is when two or more heat pump units work together to meet a building’s space heating or domestic hot water (DHW) needs.
Here, we’ll cover all you need to know about cascade systems, from the different types to their pros and cons.
What are cascade systems?
A cascade heat pump system is all about two or more heat pump units working together to meet a building’s needs.
This might involve them fulfilling different tasks at the same time – for instance, one heat pump unit might provide heating while another provides DHW.
Or, it might involve them working together to share a building’s heating and hot water demand between them.
How do cascaded systems work?
Wondering exactly how cascaded systems work?
Well, the word ‘cascade’ can mean a couple of different arrangements of heat pumps.
- A series of heat pumps joined together to produce a great amount of heat. E.g. 4x 50 kW for 200 kW. This arrangement is also known as multiplex.
- A series of heat pumps stacked together where one supplies the next in order to generate specific temperatures. There are normally two but for some applications, e.g. really high temperature, it could be three.
1. Multiplex cascade heat pumps
A multiplex arrangement involves joining together a series of heat pumps to produce a great amount of heat.
For instance, you might combine four individual 50 kW units to achieve a total of 200 kW.
These individual units are joined at a single control panel,
Through this control system, the heat and hot water needs of a building 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.
Why use a multiplex cascade heat pump?
For commercial properties, cascade heat pumps can be very useful if…
- You don’t have modulating heat pumps
- You have exceptionally high and/or fluctuating demand for heat and hot water
- You need certain levels of redundancy in the system
Many commercial properties with these kinds 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 modulating, in which case it is not necessary to cascade which is often cheaper.
Multiplex cascade heat pumps can also be good for resilience and are often used with non-modulating heat pumps to give an overall modulating output.
Resilience is a good reason to multiplex but we don’t recommend more than two and sometimes three heat pumps because the costs outweigh the benefits.
Either way, with Clade heat pumps, which are modulating, it isn’t necessary to do this.
2. Cascade for temperature
Alternatively, you can get a series of heat pumps stacked together – normally two – where each supplies the next in order to generate specific temperatures.
In this case, each heat pump uses a different cycle with a different refrigerant – one low temperature and one high temperature.
In the first, the refrigerant evaporates at a very low temperature and condenses in the heat exchanger at relatively low pressure and temperature.
This low temperature heat is used in the evaporator of the second heat pump and causes the second refrigerant to evaporate.
The second refrigerant also condenses at a relatively low pressure, keeping the heat output high.
In other words, the low temperature 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.
Why use a cascade heat pump for temperature?
The major advantage of these cascade heat pumps is that they can be used as a direct boiler replacement.
Let us explain.
Standard heat pumps typically operate at much lower temperatures than traditional boilers – usually, they’ll only reach between 44C and 55C, while traditional boilers are usually set up to operate at 80C with a 60C return temperature.
A cascade heat pump, on the other hand, can be set up to reach much more familiar temperatures.
This makes them easy to retrofit, as no emitter changes are necessary and controls can stay the same. So, the work inside the building is minimal.
Benefits of cascade systems
A cascade heat pump is a great example of the flexibility of the technology to achieve all manner of temperatures and services.
Here are a few examples of how it can benefit businesses.
- High temperatures. Cascade systems reach high temperatures, which means they’re particularly effective at providing domestic hot water.
- Easy to retrofit. Cascade systems can provide a direct replacement for traditional boilers, making them easy to retrofit without making changes to a building’s existing infrastructure.
- Meet fluctuating temperature demands. Cascade systems can supply a wide range of temperatures, from low using the first stage only to high with both stages at max. This can be used to provide weather compensated heating control in a very flexible way.
Disadvantages of cascade systems
As useful as cascade systems can be, it’s important to always consider the full life cost – and to make sure the end user understands the implications.
Here are the disadvantages to know about.
- Reduced coefficient of performance (COP). With cascade systems, two sets of compressors are run to produce the one output of heat, which means the overall system COP (coefficient of performance) is significantly less than a single stage heat pump.
- Running costs. The increased electrical load (due to two sets of compressors) will require a larger supply and increase running costs.
- Maintenance costs. The complexity of the system can also increase the operational and maintenance costs.
- Requires more space. With two or more heat pump units, cascade heat pumps take up more space.
- Costs can outweigh benefits. Adding more than two or three heat pumps to a cascade system can lead to costs outweighing the benefits.
- Not always the best option. There are often other options available to help you reach the same end goal – for instance, our new Elm heat pump provides a direct replacement for a boiler without the need to cascade.
Do Clade Provide Cascade Heat Pump Systems?
Yes! At Clade, our cascade heat pump system – the Larch – combines an R290 air source heat pump and R600a water source heat pump to offer flow and return temperatures that closely match those of traditional boiler systems.
The specifically designed pair of low temperature and high temperature heat pumps work in harmony for maximum efficiency, allowing each working fluid to work at optimal conditions.
Example cascade system installation
This example installation demonstrates a cascade system in action.
In this instance, the system is designed to deliver the full duty of 560 kW at 80/70C (in other systems, the cascade may only supply part of the duty).
The system consists of two ASHPs 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
- Total system COP of 2.1.
Download the case study for more details, or download the datasheet to learn more about the Larch.
Alternatives
As the leader in natural refrigerant heat pumps Clade has just launched the Elm which is a single stage heat pump that does the job of a cascade.
The Elm uses new compressor and refrigeration cycle technology with our own control strategy to generate 80/70 temperatures.
Just bear in mind that capacity is limited to 70KW at -5C ambient. So, for larger applications the Larch is definately the way forward.
Let’s design a heat pump system that works for you
As you can see, a cascade heat pump can help you to achieve all manner of temperatures and services.
However, there are also lots of other ways to reach your goals with heat pump technology.
At Clade, our system designers are here to help make sure you get the right system for your needs – whether that involves a cascade heat pump or one of our many commercial air source heat pumps.
Get in touch and we’ll be happy to design, manufacture and install a heat pump system bespoke to the needs of your business.