Pete Armstrong explores the interaction between hot water cylinders and heat pumps and the consequence of this on overall system Coefficient of Performance. This interaction depends on how the hot water is heated, specifically whether a plate heat exchanger or coil is used along with other critical design features.
Updated on 21 November, 2023
A hot water cylinder, in many ways, is like a battery for thermal energy.
Whether we’re talking about a traditional electric heating element or a heat pump, the low kW output of these systems cannot produce instantaneous hot water at the rate that a shower demands. Even gas boilers can struggle to keep up in larger properties. Consequently, a hot water cylinder is often required.
The way that thermal energy is transferred into a hot water cylinder can have a significant impact on the overall system efficiency.
The traditional approach has been to use a large coil of pipe (very large in the case of heat pump cylinders) to achieve good heat transfer between the wet heating system loop and the stored hot water inside. However, in recent years, several manufacturers have opted to go down the route of using a plate heat exchanger instead of a coil.
This article explains some technical details that influence system performance to explain Mixergy’s decision to use plate heat exchangers across our heat-pump-ready cylinder range.
Our reasoning can be summarised as follows:
We deal with the above themes in turn throughout the rest of this paper while also explaining why we don’t yet apply our top-up approach to heating with third-party air source heat pump cylinders, something we do with Mixergy’s integrated heat pump cylinder (iHP), where the heat pump and cylinder come as one unit.
We understand that some installers prefer the simplicity of coils over plate heat exchangers. Mixergy is continually reviewing our product portfolio, and despite the benefits associated with plate heat exchangers, we continue to review the state of the art as far as conventional coils are concerned; as we will see later in this article, not all coils are made equal!
The rest of this article focuses on the application of plate heat exchangers and coils in relation to heat transfer from a heat pump. While similar arguments can apply to a system gas boiler, the change in operating efficiency is generally more pronounced when it comes to heat pumps, which are much more sensitive to the system loop temperature.
The Coefficient of Performance (COP) of any heat pump system, when delivering hot water, is defined as the amount of thermal energy transferred to the hot water stored in the cylinder for every unit of electrical energy consumed by the heat pump.
In general, the performance of any hot water cylinder will be driven by the rate of heat transfer from the central heating loop and the hot water cylinder itself.
The following schematic shows the central heating loop in the context of the overall system, which might include a buffer tank, 3-port zone valve, and various space heating emitters (or radiators).
A heat pump’s performance is highly sensitive to both the outdoor temperature and the heating loop temperature during heating. At Mixergy, we have tested several market-leading air source heat pumps under controlled temperature conditions.
The following chart shows the range of COP values against three outdoor operating temperatures, -5oC, 0oC, and 5oC, based on 36 test points for a high-temperature heat pump running on R32 refrigerant. For each ambient temperature, we test across a range of heating system loop temperatures to understand the overall performance:
When conditions are very cold (for example, -5oC or less), the COP can drop to below 2 for system loop temperatures exceeding 50oC. Conversely, if we have a very low system loop design temperature during these conditions, the COP can exceed 2.5, an improvement of 25%.
When the temperature outside is warmer (5oC or above), the COP can comfortably exceed 4 at very low flow temperatures but will still drop precipitously towards 2.5 as the temperature exceeds 50oC and beyond.
All of this reinforces the point that it is critical to ensure that the heating system design delivers as much space heat as possible at the lowest system loop temperatures to achieve the highest energy efficiency, hence the emphasis on high surface area radiators and, where possible, underfloor heating.
Given that a hot water cylinder has to heat to 50oC and beyond during the warming-up phase, the COP will drop as it approaches the final set-point temperature. This is because the hot water cylinder effectively pins the system loop temperature down to the temperature of the water stored in the cylinder, assuming there is enough heat transfer between the system heating loop and the stored hot water inside.
In a conventional hot water cylinder, where a coil of pipework transfers heat between the system loop and the stored hot water, the coil’s surface area is critical since this will determine the temperature drop between the system loop and the stored hot water in the cylinder itself.
This is shown conceptually by the following figure, where you can imagine the thickness of the coil inside the radiator as a kind of resistor to heat flow, which develops a temperature drop between the heating loop temperature and the stored hot water. This temperature drop is very much analogous to the voltage drop across an electrical resistor when current is flowing through it:
The heat transfer resistance is determined by a combination of factors, including:
The surface area of the coil is the most often quoted parameter when it comes to heat pump cylinders; however, like many technical parameters (whether it’s camera megapixels or the fuel economy of a car), there is more nuance to this than meets the idea (as there is in the case of megapixels for cameras, where the aperture size and lens quality also makes a huge difference or a car’s fuel economy which might vary considerably depending on whether regenerative braking is available or not).
The overall surface area is very important as a critical driver of the overall heat transfer coefficient associated with the coil material and the stored water; this is clear from the following equation, which describes how effectively heat is transferred through the coil material:
Where Hmat is the overall heat transfer coefficient of the coil (or plate) material, Kmat is the material’s thermal conductivity, and Area is the surface area of the coil or plates.
This is often overlooked. Copper has a thermal conductivity of ~400W/mK, whereas stainless steel might only have a thermal conductivity of around 15W/mK. In the UK, most hot water cylinders are made of stainless steel, including the coil. However, stainless has a much higher tensile strength than copper; this allows for a much thinner material, leading to the next important parameter.
Whether a coil or plates are heating the hot water, it is essential that the system can withstand the operating pressure of the system loop (typically 3 bar or more) without bursting. Copper has a tensile strength of about 210MPa, whereas stainless steel is more than twice as strong at just over 500MPa. In the case of stainless, this allows for a much thinner material (~0.5mm as opposed to ~1.5mm) to be selected, which offsets some of the advantages of copper in terms of thermal conductivity.
Whenever a fluid exchanges heat to or from a surface, a viscous film layer occupies a few millimetres at the boundary between the fluid and the surface. This film transfer coefficient is often referred to as ‘forced convection’ and is influenced by a combination of factors, including:
In the same way that we have a film transfer coefficient between the system loop and the inside surface of the coil, there is also a film transfer coefficient on the other side between the coil/plate and the stored hot water. In the case of the coil, the film transfer coefficient is determined by the degree of natural convection between the coil and stored hot water in the cylinder. Natural convection arises because water expands and becomes lighter as it is heated.
As the water immediately surrounding the coil warms up, it expands and moves away from the coil, allowing cooler water to take its place. The rate of natural convection is influenced by several parameters, including the spacing between the coils in the hot water cylinder.
Some manufacturers will achieve a high coil surface area by packing the coils tightly together; this, however, reduces the rate at which natural convection can occur and diminishes the overall system COP as a result.
It is here that a plate heat exchanger has an advantage over a coil.
In general, natural convection is less effective in comparison to forced convection. The forced convection achieved by a circulating pump and plate heat exchanger in the Mixergy cylinder greatly improves the film transfer between the plates and the stored water compared to the coil. For this reason, plate heat exchangers allow phenomenal amounts of heat transfer inside a small combi-boiler, where 30kW+ of heat can be easily transferred between the burner and the system heating loop.
Bringing all of the above factors together, we see that our conceptual model of Figure 3 is, in fact, more complex and perhaps better represented by the following figure:
In practice, hot water cylinders are nearly always heating from a partial state of charge. For a cylinder with a coil, heating from a partial state of charge leads to what we call ‘coil lock-out’. When someone takes hot water out of the tank, it is replenished by cold water into the bottom, establishing a thermocline (separation between hot and cold). Above the thermocline, the coil transfers heat very badly as there is a small delta T whereas below the thermocline, coil heat transfer is much better due to the larger delta T. This is illustrated by the following figure:
With a plate heat exchanger scavenging water from the bottom of the cylinder, we are always transferring heat with the largest possible delta T during operation which means the highest COP can be delivered irrespective of the state of charge.
A plate heat exchanger can transfer heat to the stored hot water with a lower overall temperature drop to the system loop compared to the coil. This means that as the cylinder warms up, the average COP of the heat pump is improved.
In practice, whether the coil or plate wins out will depend on the relative size of the plates and the flow rate of the circulating pump versus the size of the hot water coil.
We have tested the Mixergy cylinder with our plate heat exchanger arrangement against a market-leading hot water cylinder, where we delivered an overall improvement of 10% in COP when heating during an outdoor temperature of 0oC from a temperature of 10oC to 55oC.
This test was conducted with a 150-litre Mixergy cylinder compared to a 150-litre conventional cylinder, which used a coil. This difference will diminish as hot water coils get bigger in larger tanks.
A conventional hot water coil transfers heat between the system heating loop and the stored hot water via natural convection around the immersed coils in the tank. Depending on the coil arrangement, a stratified temperature distribution will emerge during heating, meaning that cold water below the bottom of the coil will remain even once the tank has fully heated. This is because thermal energy mixes throughout water readily due to natural convection but conducts poorly downwards due to water’s surprisingly low thermal conductivity (~0.6W/mK).
Our approach, which involves a plate heat exchanger and circulator pump, enables us to ensure that the entire volume of the hot water cylinder is heated, as shown in Figure 7.
The benefits of our heating arrangement (shown on the right-hand side of Figure 7) is that:
This second point is important regarding the growth of Legionella bacteria, a pathogen that develops flu-like symptoms in people with compromised immune systems (such as the elderly). The Mixergy tank employs a sensor that extends to the very bottom of the cylinder, verifying that the temperature goes beyond 50oC (the point at which Legionella bacteria starts to die off).
As far as conventional air source or ground source outboard heat pumps are concerned, our approach to heating is similar to a conventional cylinder in that the entire volume of hot water is heated in ‘one go.’ The reason for this can be made clear by considering the trajectory of COP during heating, as shown by the following figure:
In the case of heating the whole tank from cold to warm, we benefit from the COP being much higher in the earlier stages of heating until the final set point temperature has been attained.
On the other hand, heating instantaneously at the set-point temperature will result in a lower COP from the beginning. The impact of this in the case depicted by Figure 9 is that the COP would have dropped from about 2.9 in the case of the standard approach to heating down to about 2.2 during instantaneous heating. This reduction in COP would more or less wipe out the benefits of reduced heat losses.
Our integrated heat pump cylinder (iHP) can alternate between top-up heating at a lower COP and whole tank heating at a higher COP to deliver fast reheat when the tank is near empty. We can do this because we have tight control over the heat pump in the iHP unit. Unfortunately, third-party heat pumps have different control characteristics, so we are currently unable to deliver a reliable top-up experience with these systems.
However, we can provide a quick top-up of heat with our backup immersion at the top of the hot water cylinder. If the tank, for whatever reason, does run out of hot water, this provides the customer with the opportunity to quickly boost the cylinder to a useful temperature within 20 minutes rather than waiting an hour or more, depending on the size of the heat pump.
In summary, Mixergy’s approach to heating via plate heat exchangers alongside other design features associated with our heat-pump-ready cylinder, delivers the following benefits:
The science behind heat transfer and the various features associated with different coils is a deep subject that continues to evolve. Many manufacturers will quote a coil’s ‘surface area’ as a single metric.
As we have seen, surface area alone doesn’t account for everything. Coil thickness, material conductivity, presence of corrugations or fins, all make a big difference.
We would like to see a move away from simply reporting surface areas alone. In our view, the best metric is the overall heat transfer performance, which can be expressed in different ways. For large industrial heat exchange systems, a parameter called the Logarithmic Mean Temperature Difference is often used (LTMD). Whether an LTMD value or simply ‘heat transfer coefficient’ is used in the future, this would allow an ‘apples with apples’ comparison to remove confusion from a complex subject!