As my economics lecturer used to say when I was an Undergraduate at Aston University back in the late 1980s – “You makes your choice. You pays your money”.
Aside from his questionable grammar this phrase comes to mind when I’m talking to design consultants and building owners about selecting from the range of commercial heating products that are on offer today.
The Shift Towards Air Source Heat Pumps
Aside from the clear direction towards electricity as the input ‘fuel’ for space heating, by which I mean the seemingly irrepressible drive to select Air Source Heat Pumps over Gas Boilers for almost every application, there are also wide range of considerations when selecting a heat pump that come into play.
Comparing Boilers and Air Source Heat Pumps
Unlike modern condensing gas boilers which generally share similar operating efficiency performances (around 95% Part L Seasonal Efficiency) and output temperatures (generally up to around 90°C) it is a totally different picture when it comes to the published efficiencies of available Air Source Heat pumps.
Essentially all these machines operate in the same way by transferring heat from a compressed refrigerant into a wet system across a plate heat exchanger. However, there is a wide variation in how efficiently this happens and, as a result, a wide variation in the running costs of different machines on the market.
STREBEL’s Approach to Efficient Heating Technology
At STREBEL, we have always taken the view that highly efficient heating technology should be at the forefront of product design. There are always going to be different manufactured goods on the market, each with their own idiosyncrasies, but fundamentally one performance characteristic stands out to us when we consider bringing a product to market – that it operates with a low cost and delivers the required amount of useful heat at the required temperature.
This has never been more pertinent than today with the elevated cost of energy; specifically with the drive towards the electrification of heat with its’ significantly higher kWh cost compared to natural gas.
Building Regulations and the Role of SCOP
With the recent revisions of the Building Regulations driving a reduction in system operating temperatures down to a maximum of 55°C there is a clear direction towards the application of Air Source Heat Pumps as these operate with a higher Coefficient of Performance (COP) at lower flow temperatures.
What Is SCOP?
However, a more accurate and useful characteristic of operational performance is the seasonally adjusted annual average efficiency measure. In the case of air source heat pumps this is the Seasonal Coefficient of Performance (SCOP) value. Simply put the higher the SCOP of the machine the less energy it will use, and therefore the lower the electricity bill will be.
How Product Design Influences SCOP
Product design engineers have several ways to achieve high SCOP when designing heat pumps. Surface area and air gap spacing in heat exchanger design are critical, as is selection of correctly sized fans. Paramount, however, is the matching of compressors with a refrigerant to optimise both thermal power and temperature in the most efficient manner.
Refrigerants and Their Impact
Different refrigerants, dependant on their density and composition, possess different thermal conductivity characteristics when matched with compressors to offer a wide range of temperature outputs and thermal efficiencies.
Notwithstanding this each refrigerant will have a Global Warming Potential factor which allows comparison of the global warming impact of different gases. In recent years there has been increasing pressure to reduce the GWP impact of HFC gases and this trend is set to continue.
Balancing Costs, Efficiency and Energy Sources
Clearly all building operators will favour design changes that encourage an improvement in efficiency as this will drive down running costs. With energy costs front and central for the foreseeable future installing technology that reduces running costs is the fundamental challenge for designers of air source heat pumps which, let us not forget, cost significantly more than modern gas boilers to buy, and run on an energy source, electricity, that is also between three and four times more expensive than natural gas per kWh.
Air Source Heat Pumps and Carbon Reduction
Unquestionably electricity is considerably less carbon intense than it was decades ago. Recent reductions in the SAP carbon intensity rating of electricity demonstrates this.
With the direction of additional electricity generation moving towards low carbon means such as nuclear and wind turbines installing heat pumps now is future proofing a reduction in carbon emissions into the next decades.
That is not to say that green hydrogen may not have a role to play in decarbonising heat and hot water, but as this fuel is not readily available to scale at present it is not a real alternative for decisions today.
The Golden Rule: Choose the Best SCOP for Your Project
Therefore, when selecting an ASHP we, at STREBEL, consider the golden rule is to select the machine that gives the highest SCOP performance at the output and temperature required for each project. After all machines that use less energy not only cost less to run but also, by definition, have a lower operational emissions footprint by the fact they use less energy in the first place.
SCOP at High Temperature Applications
If the project requires a higher than usual temperature – say up to 68°C for a retrofit installation – then it is key to consider the SCOP at a high temperature. Our S-ASX-VP machine, with a SCOP of 3.09 at 65°C has been designed for just such applications combining twin scroll compressors with an additional vapour and liquid injection system and a R410A refrigerant circuit.
SCOP at Low Temperature Applications
Alternatively, if the project has been designed to operate at a lower temperature – say up to 55°C for a new build – then it is more appropriate to consider the SCOP at that level. Our S-ASX-LP model, with a SCOP of 3.37 at 55°C, is the optimal selection matching twin scroll compressors with a low GWP R454B refrigerant in this case.
Transparency in Data for Better Decisions
As SCOP, outputs and achievable consistent outlet temperatures vary with different compressor and refrigerant combinations so, of course, do the GWP ratings of the chosen refrigerants as well as the embodied carbon numbers for each machine.
Making all such information accessible and easy to read is a key responsibility of the manufacturer. Publication of TM65 data as well as full temperature ranges of SCOP and COP conditions is paramount to the selection process. After all, if you don’t have the information then it’s impossible to make an informed decision.
Like my old Economics prof used to say “You makes your choice. You pays your money”
