The only way to heat high demand hot water


19 June 2023

Richard Warry, Managing Director of ICG Heat Pumps looks at the growing commercial heat pump market

Heat pumps are becoming the go to solution for heating and hot water in commercial buildings. This is largely stimulated through the energy & carbon targets required to achieve building regulation compliance, the public sector decarbonisation scheme (grant scheme for the public sector) and, end users desire to achieve their zero carbon/net zero targets.

When it comes to high demand hot water, we at ICG Heat Pumps always recommend C02 heat pumps. Applied correctly, they will be 5-6 times lower carbon than gas boilers and circa 20% cheaper to run. In comparison with conventional heat pumps, they will be circa 40% lower in carbon and running cost.


What is a C02 Heat Pump?

Like conventional heat pumps, C02 heat pumps absorb heat energy from the air (or water) and use this and pressure created by compressors to evaporate a refrigerant that in turn is used to transfer heat energy to water.

It is important to be clear that a C02 heat pump is a heat pump which uses C02 (R744) as the heat carrying refrigerant. Those from outside the industry will have heard “heat pumps” and “C02 ” in the same sentence. We have been asked before; “aren’t all heat pumps C02 heat pumps?” All heat pumps will help reduce operational carbon emissions; the heat pumps we discuss here have C02 in them.


Achieving the best efficiency

When selecting a heat pump solution for any commercial application, many things should be considered to ensure an effective and efficient outcome. To achieve the lowest energy, lowest carbon and lowest running cost solution, it’s important to understand that with conventional heat pumps, the higher the water flow temperature that is required, the lower the efficiency will be and vice versa.

This is true for all heat pumps, although much less applicable in relation to heat pumps which use C02 (R744) as the refrigerant. C02 (R744) heat pumps efficiency is much less effected by the flow temperature, whether targeting 50,60,70 or 80˚C the efficiency does not reduce much.

What impacts the efficiency of a C02 heat pump most is return water temperature. To give a good efficiency, the return temperature needs to be below 40˚C – this will result in efficiencies/SCOP’s (seasonal coefficient of performance) circa 3. To give the best efficiency, the return needs to be below 31˚ C most of the time, this will result in SCOP’s circa 3.7 – 4.

Ensuring a low return temperature, requires a system to be configured and controlled very differently than a conventional heat pump hot water system. Using a number of buffers (2-3) piped in series will help increase the likelihood that the return temperature stays low. Monitoring the temperature and operating the heat pump and water circulation pumps at the right time are crucial and this is where contractors and designers often get it wrong. We service other suppliers’ equipment and have seen many C02 systems in the field that are returning poor efficiency and this is usually due to the return temperature being too high.


A challenge for the trade

It seems that the complexity is a real barrier and so we’ve chosen to provide a support solution for C02 systems. We fully design, supply and install the parts of the system which are usually a challenge for the trade. The only connections that the M&E contractor needs to make are; mains cold feed and secondary return and flow to the building – the same as what they would do with a boiler.

Our controls and hydraulics control stratification in a single buffer vessel and will ensure that the return temperature to the heat pump is always in the sweet spot which gives good efficiency.

Why is it that the return temperature effects efficiency most?

I’ve tried to simplify this bit so hopefully anyone can understand it. Let’s go back to our school science lesson for a moment.


To understand why C02 ’s efficiency is most effected by return water temperature, we need to understand/remind ourselves of two relevant bits of science:

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  1. Before we change the temperature of any matter, we need to either dry it (before heating it) or hydrate if we intend to reduce its temperature. This removal, or absorption of moisture is described as latent heating or cooling. This part of the state change is not measurable on a thermometer. Once dried or hydrated, we can then start to affect the temperature of the matter – this part of the state change is termed as sensible heating and cooling – this is where we see temperature change on a thermometer.
  2. Most of us know that all matter can exist as a solid, liquid or gas. What is less known is that there is a 4th state, which is known as the plasma state. If we continue to heat and pressurise any matter, it will eventually transform to a plasma. Plasma has no latent property (there is no moisture in it) and so any increase in pressure or absorption of energy will result in an increase in temperature.

Even though 99.9% of the universe is plasma there isn’t much on earth that exists in this state. The sun and stars we see at night are all plasma. Most matter needs to be heated to an extremely high temperature and/or be held under extreme pressure to achieve plasma state and so there are few tangible everyday examples, metal being arc welded is in plasma state, we also see plasma in neon and fluorescent lighting tubes, but that is about it.

The uniqueness of C02 is that it can transform form a gas to a plasma at only 31˚ C, if pressurised to 73Bar. The point that it changes from a gas to a plasma is called this the critical point and so we describe the plasma as trans-critical C02 .

In heat pump systems, C02 is in transcritical (plasma) state after discharge from the compressor, we aim to keep it in this form for the entirety of the heat transfer. If the heat sink (the return water) is below 31˚C, we can transfer all of the energy in the C02 without dropping the system pressure. Once the heat is transferred, we can drop the pressure just below the critical point. By doing so the refrigerant condenses and then can absorb heat in the evaporator and then needs just a little compressor power to turn the refrigerant trans-critical again. 

As a plasma, C02 does not have any latent property so any energy transferred to or from it, results in sensible heat gain/ loss, directly effecting the temperature of the C02. This means that only a small amount of compressor power can increase the refrigerant temperature from 31˚C to 50, 60, 70 or 80˚C.


Other environmental benefits of C02
We have predominantly used synthetic refrigerants in refrigeration, chiller, heat pump and AC systems for the last century. We started with CFC’s and HCFC’s but after
using them for decades, we discovered that they had ozone depleting potential so, we banned them, and replaced with HFC’s. After using HFC’s for a while, we discovered that they had high global warming potential. From 1st January 2015, due to F-gas regulations, we started a phase down of HFCs and instigated bans on use of the refrigerants in new equipment within certain sectors.

A recent European parliament vote means that the HFC phase down is very likely to get accelerated. The proposed road map would mean that HFC’s would be banned in most HVAC equipment by 2030 and in many as early as 2026/2028. What we know for sure, is that the use of HFC refrigerants in HVAC equipment is on bided time and that sooner or later they will be almost completely banned or phased out.

Another family of synthetic refrigerants (HFO’s) were introduced to the market about 10 years ago. Introduced due to their relatively low global warming potential, they have been championed as a replacement for HFC’s in many types of HVAC equipment. However, it is now known that when these chemicals are released to atmosphere they break down and create high levels of trifluoroacetic acids (TFA). Although naturally occurring, many universities believe that the use of HFO refrigerants is contributing to increased levels of TFA in the atmosphere, which in turn is getting into the artic ice cores and our lakes and rivers. There is growing concern that this could contaminate our drinking water and eventually cause damage to human health.


Synthetic refrigerants not good
So, for a number of reasons, we can
conclude that all current synthetic refrigerants are not good. If we look at history and the current situation, we wouldn’t be being over dramatic to suggest that it is more than possible that all synthetic refrigerants will either be banned or their use severally limited to applications which are critical and can’t use anything else, either sooner or later. 

C02 (R744) is classed as a natural refrigerant, it has zero ozone depleting potential, it has no PFSA chemicals (Perfluoroalkyl-and polyfluoroalkyl substances) and it has a global warming potential (GWP) of 1.

C02 heat pumps are the ultimate solution for DHW production, their use will result in reduced C02 emissions, lower running costs and use a refrigerant which is truly future proof.

ICG Heat Pumps are the only independent, specialist supplier of commercial heat pumps in the UK.

We offer low GWP and natural refrigerant heat pump solutions for all applications.