Heat Pumps: 16% COP Gain Earns Industry Award for Refra and Siemens

Author: Steed Webzell

Absolutely Chilled and Refra wins an Award at National ACR & Heat Pump Awards © ACR Journal

Refra air-to-water (A2W) heat pumps, featuring the MVL702 magnetic expansion valve from Siemens, are able to demonstrate clear efficiency gains compared to competing systems. A comprehensive test involving a Refra reversible propane (R290) heat pump with two identical circuits housing different valve technologies – the patented Siemens valve and a traditional stepper valve – demonstrate that the magnetic expansion valve helps the heat pump deliver a coefficient of performance (COP) over 16% higher. This outcome is of significant interest for commercial and industrial heat pump users as it could equate to a saving in energy consumption of thousands of pounds/euros every year (depending on model, operating conditions and electricity price). Such is the performance of the system that it has secured a prestigious industry award in the UK for Best Commercial Heat Pump Product.

The challenge of high energy prices is pushing heat pump OEMs to innovate their portfolio towards more efficient solutions. Heat pumps, the demand for which is soaring across Europe, are a more environmentally responsible alternative to traditional boilers, offering the potential for users to benefit from significant savings, but only with astute heat pump selection.

Refra  identified a way to increase the efficiency of its reversible propane heat pumps by integrating Siemens’ proprietary MVL702 magnetic expansion valve. The valve rapidly and accurately adjusts the quantity of liquid refrigerant in the evaporator, allowing Refra heat pumps to achieve considerably more heating/cooling power than those featuring a stepper valve. Taking just 1 second to transition from closed to fully open, Siemens says its MVL702 is 10 times faster than conventional stepper valve solutions. This capability is thanks to the linear movement provided by magnetic force, rather than the far slower spindle-turning technology of a stepper valve. The mechanical clearances prevalent in spindle-turning technology lead to less accurate valve opening positions.

Refra production manager, holding a new Siemens magnetic expansion valve. © Ilona Brant Pavšukova

Long-Term Cost Savings with Precise Superheat Control

Key to the success of the MVL702 is its ability to reduce the superheat value by about 30%, from 9K to 6K for example, by reacting very quickly to load changes. Compared with stepper valve technology, this superheat reduction can be even more significant in certain applications. Low superheat leads to efficient evaporator operation. It reduces temperature difference across heat exchanger, resulting in lower temperature lift of the refrigeration cycle.

Rapid and precise valve response is particularly useful when the application features many load changes, perhaps when commercial or industrial buildings experience large shifts in external temperature from night to day and applications with variable load profiles. For example such as conference rooms, where  the sudden introduction of people can necessitate cooling, with demand switching to heating when people exit. The magnetic expansion valve allows the heat pump to react efficiently to specific climate control demands.

These thermodynamic improvements are not only theoretical but also have practical implications that translate into significant financial value. The same heat pump with the enhanced superheat circuit could potentially achieve up to 14% less energy consumption, while simultaneously providing a 21% increase in heating capacity. These results underscore the practical benefits and economic advantages of the new Siemens valve in enhancing the overall performance of Refra heat pumps, which deliver best-in-class efficiency.

Refra customers can expect a more eco-friendly and sustainable solution that not only aligns with their energy-efficiency goals but also reduces costs without compromising performance. This means the customers can enjoy improved comfort levels while potentially reducing their overall energy expenditure.

Of course, actual energy savings are very much application dependent, but Siemens is prepared to put forward an estimate of what end users might expect.

“If we talk about a 300 kW system featuring our magnetic expansion valve, energy savings will likely be in the region of €300 per month,” suggests Richard Bork, Product Manager Refrigeration Valves at Siemens. “These calculations are based on 3500 full-load operating hours per year, an electricity price of €0.20 per kWh and 3K lower superheat values. The savings could of course be more or less, depending on factors such as the control quality of the existing stepper valve solution, exact operating conditions, load profile and defrosting needs,  but it provides a good idea of the saving potential.”

Refra Ignis Heat Pump during a field test in Lithuania © Ilona Brant Pavšukova

Field Test in Real Ambient temperature Conditions

Claiming a product is efficient and proving it are two different things, which is why a comprehensive real-condition, real-time test was carried out by Refra in Lithuania, involving a Refra IGNIS reversible heat pump featuring two circuits: one with the Siemens valve with another featuring a bipolar stepper valve of another brand. The circuit cooling capacity is -6/50°C 21.9 kW, while heating capacity is 31.1 kW. Both circuits feature an identical/symmetrical design.

The trial took place under specific conditions: 50°C heating set point; 30-32°C supply water temperature; and -6°C outdoor temperature. Both circuits operated at 100% capacity due to the inability to reach the set point, allowing the assessment of valve performance in challenging scenarios.

Recorded data from the test was analysed by splitting the investigated time period into 7-8 identical time frames randomly. This approach aimed to equalise deviations in fluctuating process values, ensuring reliable mean values for assessment. For calculation purposes it was necessary to make some assumptions and simplifications that would allow comprehensive analysis of the heat pump’s performance and efficiency under controlled conditions: isentropic efficiency of 0.8; pressure drops assumed as 0 bar in the evaporator, condenser, suction and discharge lines; sensor tolerance not considered; and characteristics for the compressor in each circuit maintained a consistent 35.5 m³/h volume flow with constant volumetric efficiency.

Siemens MVL702 magnetic expansion valves. © Ilona Brant Pavšukova

Side-by-Side Valve Comparison Reveals Clear Performance Advantage

The results of the analysis clearly demonstrate the profound impact of superheat control on heat pump efficiency. Lower superheat values, influenced by SH control, lead to a higher evaporating temperature, resulting in elevated COP values and reduced defrost cycles.

In particular, when comparing the Refra heat pump circuit featuring the new Siemens valve with the stepper valve circuit, there is a noteworthy increase in COP by up to 8.3% in heating mode. Additionally, a direct comparison considering the entire heating cycle (including defrost mode), reveals an even more substantial increase in COP. Due to the longer heating mode of the magnetic expansion valve circuit, COP rises by up to 16.4% compared with the stepper valve circuit.

The data record and analysis for the stepper valve circuit is shown below:

Data record from internal performance report © Siemens

After defrost transition, the superheat setpoint (2) starts from 7 K before increasing (due to high fluctuation) to 10 K. The best achieved control deviation of superheat value is +0.3/-0.6 K, while the evaporating temperature (1) shifts from -16°C down to -20°C and triggers defrost transition. The average fluctuation of suction gas temperature is ±0.5 K (3). Notably, the total heating period for the stepper valve circuit is 1:01:12 hours.

The contrasting data record and analysis for the circuit featuring the MVL702 magnetic expansion valve, which has a completely different operating principal to a stepper valve, is shown below:

Data record from internal performance report © Siemens

This circuit gets much faster into stable conditions after defrost transition as indicated by the smooth (rather than wavy) graph lines. The superheat setpoint (2) begins at 6 K prior to increasing within minutes to 6.2 K with a maximal overshoot of 0.4 K. The average control deviation of the superheat value remains constant at +0.0/-0.2 K, while the evaporating temperature (1) shifts due to frost from -13.8°C down to -20°C and triggers defrost transition. The average fluctuation of suction gas temperature is again ±0.5 K (3). As a result of this performance, the total heating period (without defrosting) provided by the Refra heat pump with Siemens valve more than doubles to 2:11:00 hours.

Minimising Defrost Time Unlocks Greater Energy Savings

Another major advantage of using magnetic expansion valve technology in Refra A2W heat pumps involves the defrost function. A lower superheat makes it permissible to operate the system at higher evaporation temperatures, especially in part load, and make use of heat transfer from the evaporator. The result? Far less icing on the evaporator in air temperatures of ≤0°C. Less icing means less defrosting. All heat pumps use energy to defrost, so reducing the time needed for defrosting – and the quantity of defrost cycles – is of huge benefit to commercial and industrial users.

In the test, the heat pump circuit with the stepper valve could operate its heating cycle for 3,787 seconds (around 63 minutes) without running a defrost cycle. In contrast, the circuit with the MVL702 magnetic expansion valve was able to operate for 7,961 seconds (approximately 133 minutes). It demonstrates the potential to save a little over 50% in energy consumption. Moreover, instead of defrosting, the system can use the saved defrost time to perform heating. This capability can have the effect of doubling the energy-efficiency performance of the heat pump.

Heat pump production line at Refra factory in Lithuania © Photo by Ilona Brant Pavšukova

National Award Signals Refra’s Breakthrough in the UK Market

Recognising the potential game-changing advance of Refra reversible heat pumps with Siemens magnetic expansion valve technology has led to a prestigious award. Absolutely Chilled Ltd, the exclusive UK distributor for Refra heat pumps since 2018, has been named as the winner in the Commercial Heat Pump Product category at the National ACR & Heat Pump Awards (awarded by ACR Journal and sponsored by Heat Pumps Today).

The award, presented at a high-profile ceremony in Manchester, UK, reflects Refra’s long-standing commitment to environmentally responsible solutions that stretches back to 2011. As the first field test partner and early adopter of the Siemens magnetic expansion valve for natural refrigerants, the company is leading the way towards a more eco-friendly future. The entire Standard Line of Refra reversible A2W propane heat pumps and propane chillers now features the MVL702 valve.

Shane McKenzie, Business Development Director at Absolutely Chilled, says: “Winning the award is an amazing achievement as it gives us a great platform to introduce Refra to many new clients backed by a prestigious industry accolade for efficiency. It gives us one of the most important points when promoting Refra to the market: credibility. With this award and the tangible efficiency gains demonstrated by comprehensive tests, why would you consider anything else? All other heat pump manufacturers are now playing catch-up.”

The UK is expected to be a strong market for Refra heat pump technology after the country’s government set aside up to £20 billion to support the early deployment of carbon capture, usage and storage (CCUS). The Public Sector Decarbonisation Scheme (PSDS) is part of this initiative, aiming to reduce CO2 emissions from public buildings by 75% by 2037 (compared with a 2017 baseline). In fact, ten DSEAR-compliant (corresponding to the European EN378 standard) Refra propane heat pumps with Siemens MVL702 valve and inverter-driven reciprocating compressors have been already installed in four schools throughout the country.

“The adoption of Refra heat pumps by project engineers, consultants and contractors in new buildings, or the replacement of traditional boilers in existing premises, is what I would call a no-brainer,” says Shane McKenzie. “The irrefutable data from Refra/Siemens speaks for itself, with a further compelling argument in the form of natural refrigerant, which Refra has been working with for over a decade.”

Richard Bork has the final word: “We decided to work with Refra as one of our initial field-test partners and the first serial MVL702 adopter because of our long-standing and successful professional relationship, which is based on trust. Refra is also very flexible in its development programmes, reacting quickly and providing feedback, which helps expedite the process of bringing a new product such as the MVL702 to market. Refra was the perfect early adoption partner and we really appreciate their co-operation.”

Absolutely Chilled and Refra wins an Award at National ACR & Heat Pump Awards © ACR Journal