When is a chiller not a chiller?

The answer, of course, is when it’s a heat pump, writes Garry Broadbent of Pure Thermal. On a serious note, this question is one which is becoming increasingly relevant as energy efficiency and carbon reduction are now becoming the key specification drivers.

Garry Broadbent of Pure Thermal

To put this into context, to date the repurposing of waste heat from cooling has not been the highest design priority where retrofit heat decarbonisation projects are concerned. 

There appears to be three main reasons for this: 

1. The relatively low temperature of heat that can be generated from a heat pump that has 4-pipe functionality (4-pipe chiller) being only able to deliver a maximum output temperature of circa 60°C
2. The perceived integration issues associated with recycling heat from a cooling system that may vary in output according to cooling duty at any time
3. Due to the early stage nature of retrofit heat pumps, the funding that has been made available by the PSDS scheme has not in general encouraged integration with cooling systems.

These three factors have effectively combined to discourage more general repurposing and recycling of waste heat from cooling a building or process. 

However, it’s becoming very clear that industry is now looking to satisfy higher temperature retrofit heat pump projects with systems that deliver an annual COP in excess of 3.0 in order to combat the negative effect on heat pump application caused by the high cost of electricity when compared to gas.

Clearly this level of efficiency is not possible with an air source-only approach, where a ‘must-have’ high temperature retrofit application is considered, but this level of efficiency can be achieved by utilising the chilled water return as the heat pump source rather than air. 

This makes us consider that we have perhaps been operating one dimensionally for the past 30 years or so. On this basis, identifying an application with a 52-week-per-year cooling demand has historically been viewed as a very attractive opportunity to apply a well performing chiller to deliver the most efficient cooling performance.

But now this opportunity can also be more widely viewed as an opportunity to provide high efficiency decarbonised heat as well as reliable cooling.

It’s not as if this isn’t being done already with two stage/cascade air source and water source systems. However, these cascade systems can be challenging for a user to accept due to the availability of space for the plant, the perceived duplication of heat pumps and the associated cost of installation.

However, we are now seeing technology moving forward and the challenges/barriers noted above can be removed by applying single stage chiller heat pumps that are able to comfortably deliver cooling. Crucially, these units are able to deliver heat outputs of 80°C even in external temperatures below -15°C.

This means that there is now the possibility to install a straightforward and easy to integrate single unit that will deliver simultaneous 4-pipe functionality providing both CHW and LTHW outputs.

So how can a chiller-heat pump be applied?

The big opportunity here is to consider retrofit efficiency and carbon reduction focused projects where the integration of the return circuit of a chilled water, or water glycol, cooling system is used as the source for a heat pump instead of air source.

If this type of application can be identified, then this represents the highest efficiency heat pump opportunity.

With this in mind, chiller replacement projects offer real potential but also applications where a chiller is not due for replacement are also attractive. 

Here the chiller-heat pump can be integrated within the chilled water return and, if practical, can provide a COP of over 5 which means that this type of application is by far the most efficient heat pump that the building or process can benefit from.

As an example, the Single Stage Pure Thermal A80 range is able to provide a cooling output down to -10°C and deliver a simultaneous heating output up to 80°C with full 4-pipe modulating functionality. 

This provides an ideal option to replace a chiller in order take full advantage of any winter cooling load to simultaneously deliver 7°C cooling and 80°C heat outputs with a TER of 3.6.
The table below shows the A80 300.2 using a 12°C chilled water return as its source.

However, with these retrofit applications it is important to note that if the input power to the existing chiller is deducted, because the existing chiller has been displaced by the chiller-heat pump, then the TER would then be 5.5.

Pure Thermal A80 300.2  Chiller-Heat Pump (Single Stage 4-pipe simultaneous functionality)
Cooling capacity	kWt	134.2
Heating capacity	kWt	235.7
Power input	kWe	101
TER	W/W	3.7
TER (chiller input power corrected)	W/W	5.5
Condenser
Fluid type	-	WATER
Medium inlet/outlet temperature	°C	70/80
Medium flow rate	m3/h	20.7
Condenser pressure drops	kPa	7.4
Evaporator
Fluid type	-	WATER
Medium inlet/outlet temperature	°C	12/7
Medium flow rate	m3/h	23
Evaporator pressure drops	kPa	10
Refrigerant (Natural/Hydrocarbon)	HC	R290

 

To enable industry to move into these new areas of application it’s clear that technology development, or more to the point compressor technology development, will play a key part in developing machines such as this type of 80°C single stage chiller-heat pump. 

The 300.2, for example, unit utilises a purpose designed higher refrigerant operating pressure R290 reciprocating compressor that has been designed to make higher temperature operation possible. In heating mode the 300.2 is capable of operating with a 80°C output temperature in external temperatures as low as -20°C which does to tell you something about the credentials of these high specification units.

Here we see the difference between a standard chiller-heat pump with a 60°C maximum output temperature and this 80°C high temperature reciprocating compressor unit that has an operating envelope that genuinely matches the demands of a high temperature heat decarbonisation project.

In terms of both heating and cooling, the development of technology is certainly moving forward at a pace to match the demands of our fast-developing market. 

Based around maximising input power to deliver the most efficient levels of performance this example typifies where chillers and heat pumps can be optimised to deliver a combination of performance that has not been seen before in a single Monobloc unit configuration.

Across industry new ways to optimise heat pump efficiency are being developed alongside existing methods of applying heat pumps using direct refrigerant systems to provide heating and cooling, such as VRF/V or simple split AC units that are now commonly also referred to as air-to-air heat pumps.

But it’s important to note that refrigerant drivers, F-Gas phase out and GWP, mean that the need to reduce refrigerant system volume is also on the radar of a specifier/designer in terms of both practically minimising the risk of leaks and reducing TM65 lifetime carbon emissions.

Usually on a retrofit if a wet heating system is employed then it will be best to first visit combined cooling and heating as per the A80 type unit with R290 refrigerant before considering a new direct refrigerant retrofit system.

Hence the introduction of 80°C 4-pipe chiller-heat pumps creates more options where the priority is to provide a system that makes the strongest commercial case by delivering a TER 3.6 or TER 5.5 if the input power to an existing chiller is displaced. 

In summary this represents a different way to view a chiller application.

Even if the simultaneous winter cooling load, that will act as the source for the heat pump, only represents 25% of the total buildings heating demand, then the chiller-heat pump should still be considered.

Why? Because this 25% will be the most efficient decarbonised heat available to the building and represents the most efficient method of applying a heat pump within that building. This demonstrates why reviewing the waste heat from chillers should always be the first step when considering the decarbonisation of a building or process/production facility.