Warming up to the Heat Pump: Sustainable Heat Transfer Part 2
- jamesalexanderfawc
- Feb 21, 2024
- 11 min read
Updated: Feb 22, 2024
Included:
As our industry moves towards decarbonisation, there have been some compelling arguments towards adopting the Heat Pump as a means of generating heat. Let’s get right into it and see why heat pumps are likely to be a feature in the modern brewery.
What is it?
Heat pumps work almost exactly the same as refrigerators. They use the same OVC cycle (Evaporation, Compression, Condensation and Expansion) to move heat energy. Only this time, instead of moving heat away, we are trying to move towards.
With a refrigerator or chiller, the heat source is something like a fermenting FV, and our heat sink is commonly the ambient air.
In the case of a heat pump, our heat source is the ambient air and our heat sink is what ever we are trying to heat up.
Therefore the key variable that distinguishes a heat pump from a refrigerator is the type of primary refrigerant that is used. If you remember from Part 1, chillers “store” heat potential energy as they move through their OVC cycle in the form of a phase change. The type of refrigerant (and its evaporation temperature) must match the temperature of the heat source. This is why you can’t use a domestic heat pump for industrial heating.
Let's have a look at the OVC cycle in relation to a heat pump.
Heat Source => Evaporate => Compress => Heat Sink => Condense => Expand
Evaporation
Since we are trying to transmit heat TO a target, we have to find the potential heat energy from elsewhere. This could be the ground, the air or a heat exhaust point (kettle chimney, compressor fan, chiller condenser fan etc.) We will use this heat to evaporate our refrigerant. Remember when we spoke about Cooling Capacity for chillers? Well this is the Heating Capacity of a heat pump. The heat pump is at maximum capacity when all the refrigerant has completely evaporated. When it comes to heat pumps, the boiling points as well as the critical point* of a refrigerant are what we need to focus on (more on this later).
*The “critical point” is the temperature at which a refrigerant transitions between its liquid and vapour phases under specific pressure conditions, beyond which the distinction between liquid and vapor phases disappears. At this point, no amount of pressure increase can cause the refrigerant to condense into a liquid. Instead, it exists as a supercritical fluid with properties of both liquids and gases. This means that the ability of latent heat transfer is lost.
Compression
Just like a chiller, the compressor is the workhorse. It increases the heat energy by increasing the pressure of the refrigerant. When it comes to heat pumps, this will be how we can elevate the heat energy to required processing temperature in the brewhouse.
Condensation
With a chiller we commonly condense the refrigerant by blowing air onto it, transferring the heat to the air. With a heat pump we want to use this transferred heat, and so condense are refrigerant on a heat sink we are trying to heat up. A main issue here is that as this heat sink heats up, it is less effective a condensing the refrigerant. Heat pump configuration needs to account allow for this (more on this later).
Expansion
Once again, the cooled (not cold) refrigerant is expanded to cool it more. This is important to get the refrigerant temperature to below the target heat source, otherwise evaporation will not occur.
Heat Pumps in action
As mentioned above, refrigerant selection is inherent to the intended purpose of the heat pump. The properties of the refrigerants we are interested in are the boiling points and critical points.
Refrigerants such as R32 or R290 are good for domestic heat pumps because they can boil at low ambient temperatures, and can reach temperature levels suitable for domestic use before it reaches its critical point.
Refrigerants such as R717 (ammonia) are ideal for food manufacturing purposes because it operates (evaporates) at a wide range of temperatures. By manipulating the pressure, and therefore the boiling point, you can apply ammonia to a wide range of applications, from low level heat, to wort boil (100°C).
Other refrigerants such as R601 (Pentane) take this further, operating/evaporating at higher temperatures, therefore allowing for higher temperature applications (steam boilers 180°C).
However, the efficiency of the heat pumps are subject to external conditions. For air source heat pumps in particular this is an issue because of varying ambient temperature. In colder seasons, the brewhouse hot side requirements will require the heat pump to operate at very low evaporating temperatures (0-10°C ambient temperature) and very high condensing temperature (100°C wort boil).
This is problem because:
It increases the demand on the compressor. Lower input temperature for a single stage heat pump will require the pressure prior to the evaporator to be very low (so the boiling temperature is low). In order to evaporate ammonia during winter temperature conditions, the pressure of the evaporator would have to be 4-6 bar. This leaves it up to the compressor to get up to operating temperature (increasing the pressure by 25 bar). This is increasing the “stretch” or compression ratio, which directly corresponds to a decrease in efficiency.
It increases the amount discharged and wasted heat. High condensation temperatures (100°C wort boil) and low evaporation temperatures (0-10°C ambient) means that a lot of heat energy is inevitably discharged. This is because in order for the heat source to evaporate the refrigerant, the temperature of the refrigerant must be lower. Transferring heat to an already hot heat sink (say a 95°C wort) will mean that not all the heat will be transferred; the rest of which will need to be discharged in order for evaporation to occur.
Colder ambient conditions hinder performance because cold temperature makes compressing the refrigerant harder and for them to maintain a consistent flow.
Heineken have got around this issue by using geothermal energy instead of the ambient air as a heat source. This means that their heat source is already hot and stable, meaning that less work is required from the compressor, less heat energy is discharged, and the heat pumps can be effectively configured. However, drilling an epic hole beneath our breweries is not practical for all of us. Instead our solution is a Cascade system.
Cascade
A Cascade heat pump configuration is a sequence of heat pumps that act on each other to up-cycle heat energy. They do this by taking advantage of the ideal evaporation temperature of different primary refrigerants; the 1st evaporating from ambient conditions, the 2nd evaporating off the 1st.
For example, the “normal boiling point” of the refrigerant R32 is -51.7°C. That’s plenty low enough to evaporate in ambient air conditions, no matter the season. However, the “critical temperature” (maximum) is only 78.1°C, and this would require a lot of work from the compressor.
Instead, this heat could be used to evaporate a second refrigerant, such as R601 (Pentane). This has a “normal boiling point” of 36°C. It also has a “critical temperature” of 196°C making it suitable to use in a steam boiler.
Using these two refrigerants with each other saves them from having to work at their full range, which basically means that the compressor is saved from working too hard. Despite the fact that there will be multiple OVC cycles running (multiple compressors), COPs for cascade heat pumps are much higher than single stage. This is a necessary step for heat pumps to be competitive with fossil fuel boilers.
COP
Similar to when we spoke about our chillers, there will be opportunities for us to reduce the load on our heat pumps compressor and increase the COP. Recalling again Part 1, we’re trying to reduce the “stretch” or compressor ratio; that’s reducing the difference between the pressure on either side of the compressor.
As mentioned above, if we use one heat pump to heat another, we can reduce the compression ratio and increase our COP.
But another heat pump is not the only source of heat in the brewhouse. There are a number of heat sources throughout the brewhouse that can emit temperatures of 36°C. If we can harness them, we may even be able to go without the 1st heat pump in a cascade system which would massively reduce the total compressor load.
We can start to see how heat pumps could fundamentally change the way we perceive energy in our brewhouse. Suddenly every heat source; fermentation, chiller fan exhaust, compressor fan exhaust, kettle chimney and even ambient temperature on hot days become potential ways in which we can increase the COP on our heat pumps, increasing efficiency and reducing the energy required to run them.
Batch Processing & Steam
The appeal of reducing COP by utilizing heat sources is enticing, but these heat sources might not be always available at the time that we need it. In fact, if batch rates are limited to once per day, then it is unlikely that we will be able to translate energy straight from a heat source to heat sink. In these conditions, thermal batteries can be used to “hold” the thermal energy in-between heat requirements. Thermal batteries operate by either sensible (heating up), latent (changing phase), or chemical (reversible chemical reaction).
Many of us will already have a kind of thermal battery currently in the brewhouse and perhaps not realize. Steam boilers utilize the phase change of steam to store and transfer heat energy via evaporation and condensation. Extending this storage capability with latent thermal batteries will allow breweries to take advantage of heat sources to reduce the COP of heat pump boilers without having to use the heat energy immediately.
Another way of looking at it is the flexibility of batch processing will actually enable businesses to take advantage of cheap energy supply (when the wind is blowing). This will be of particular interest to small breweries.
Why we need it
As our national NetZero deadline approaches, all industries should do their part to decarbonise. Once the low hanging fruit has been picked (transition to a green tariff), the carbonisation of heat and CO2 generated from fermentation will be more carbon intense aspect of many brewhouses.
Carbon Tax
In order to avoid a fine for failing to meet their Net Zero obligation, the government is likely to introduce measures to encourage decarbonisation. If a tax-at-point-of-use strategy is introduce (taxing when there is a green alternative) those still burning gas to generate heat will be hit hard.
Can’t rely on Regen
A lot of buzz has recently surrounded “regenerative” barley and its carbon offsetting ability. Although regen crops will undoubtedly play a key role is a sustainable future, they should not be used to absolve responsibility to decarbonise in-house emissions. Not only are these offsets insecure, but the numbers don’t amount to what is needed to offset the emissions from the rest of the supply chain (despite what you may have read). All industries must make individual efforts to decarbonise or we will miss our NetZero target.
Gas & Biogas
Ultimately, burning is burning, whether natural or biogas. Despite the fact that it might be an efficient use of organic resources to put your spent grain through an anaerobic digester, GHG emissions are still released into the atmosphere and so it is an inferior sustainable heat source when compared with electricity or hydrogen. As mentioned above, part of an extended carbon tax (UK ETS) may involve a tax-at-point-of-use strategy. It’s likely that this won’t distinguish between emissions associated with the burning of natural or biogas as the emissions are similar, resulting in a significant financial obstacle for anaerobic digesters.
Hydrogen
During the discussion on the decarbonisation of heat there has been a considerable amount of disagreement about whether hydrogen or electricity will be the dominant source of heat energy in the future. While I think the overall competitiveness is unhelpful, breweries must make a decision which one to adopt as it is outside of their budget to switch before the end of the technologies lifetime.
One reason why the debate is so fierce is because both technologies are lobbying the government to fund the supporting infrastructure needed. The electrical grid is not capable of supporting full migration to electrical heat via heat pumps, but hydrogen usage would require converting gas lines to handle the high pressured, low temperature conditions required to transport liquid hydrogen. On this point, it seems likely that the grid will be expanded to meet demand, purely because there is a precedent. All breweries will already have 3-phase power and our usage is 37% higher than today in 2005, proving that we can already effectively meet increased demand.
There is also some interesting lobbying being carried out by domestic ground source heat pumps company Kensa. If successful, ground source heat pumps will be regarded as part of the solution to decarbonising domestic heat. This is important because the accompanying infrastructure of ground sourced heat pumps is collective. A bore hole is drilled (similar to Heinekens) and the heat scavenged below the earths crust is shared between a street or neighbourhood. This is key because it paves the way for similar industrial ground sourced heat pump infrastructure.
The second point that needs consideration is the consistency of energy. It has been forecasted that 10-20% of green energy will not be met by our renewables due to a predominant reliance on wind energy, and as a result the resultant drop in energy provided during low winds. This then drops to another decision to either over spec wind farms to ensure a higher baseline is produced, or battery storage, but again these are not straight forward answers. Conversely, the hydrolysis of hydrogen provides an efficient way to store potential energy and provide a constant reliable source.
What it really seems to boil down though is how efficiently you can make energy from either hydrogen or heat pumps, and this highlights heat pumps at a clear advantage. This is because the production of hydrogen is “Pushing against a big inefficiency”.
A combination of electrolysis inefficiency, energy requirements of transportation, and the inefficiency of industrial combustion boilers mean we are getting close to losing 50% of the energy used to make hydrogen.
On the flip side, heat pumps are generating 300% efficiency, and will likely perform better with more favourable warmer conditions.
In conjunction to this we must also consider what Adair Turner (Chair of the Energy Transitions Commission) refers to as the “Thermodynamical Technological Frontier.” Electrolyses consuming 45kW to produce 1kg of hydrogen are thought to be at the limit of possibility. Heat Pumps however theoretically could achieve COPs of 15 (that 1500% efficiency) so we are a long way off their potential.
“I don't see hydrogen competing on the cost directly with electricity, electricity that is produced from renewables, because renewables today are the cheapest energy source that we have.” Dr Silvia Madeddu
There is also the health & safety consideration, the explosive risk and potential NOx production associated with hydrogen, however there is no reason to assume that breweries will be unable to safely manage this as there is a precedent with ammonia and high pressure apparatus.
Ultimately it seems that even though hydrogen is likely to play an important role in ensuring a consistent supply, this would more efficiently be used in a power station rather than individual businesses.
Perhaps most importantly though, is that heat pump technologies are available NOW. Waiting for hydrogen infrastructure to catch up with potential demand may take years. Conversely, as illustrated by Dr Maddedu paper; 78% of industry could be electrified by technologies that are “fully developed” & “established in industry”. Add to this technology at a lower stage of development, this increased to 99% of demand.
These are technologies that are already available. More will come, the COPs will increase and the price will drop.
Mitsubishi Electric: COP 4, 171°C
Heaton: COP 5.4, 80-200°C
SPH: COP 4.7, 135°C
ECOP: COP 4-7, 150°C
Conclusion
We should all be at the very least be considering options to decarbonise our heat. Some of us will be looking to imminently upgrade our kit due to expansion or maintenance, whilst other maybe be looking further ahead.
As one of the most carbon intensive aspects of many of our Scope 1, it is essential that we start to engage with decarbonising our heat if we are to reach our NetZero goals.
Granted, this is not low hanging fruit. It will require capital. Nor is this the largest slice of your businesses carbon footprint pie. But it is an area that you have complete control over.
Heat pumps look to provide another example of how building sustainability into your business will both save the planet as well as keeping pounds in your pocket.
I hope this article has been able to familiarise and illuminate the possibility of heat pumps in the brewhouse. As always, let me know if I can help.
Good luck out there,
James
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