Why heat pump cylinder design matters

Jason Hobson, divisional director at McDonald Water Storage, explains how the design of heat pump cylinders influences their performance and flexibility. 

Jason Hobson
Divisional director McDonald Engineering

In 2020, the Prime Minister set out an ambitious ten-point plan for a green industrial revolution to create and support up to 250,000 British jobs, with part of this plan to make our homes greener, warmer and more energy efficient.

Key to this plan was the drive towards electrification and ambitious targets to remove gas from new build homes from 2025, which clearly has a knock-on effect to what happens in the retro fit market. Whilst there is ongoing debate over technologies such has hydrogen and hydrogen ready boilers, the industry feels that heat pump technology will be a key driver in achieving carbon reduction.

The increase of ‘greener’ heating technology will see heat pumps become one of the preferred methods, which will require hot water storage to maximise their potential benefit. However, a standard indirectly heated cylinder will not work with the heat pump input because the heat exchanger is simply not large enough to extract sufficient heat from the lower temperatures supplied.

Maximising the Cylinder Performance
A traditional gas boiler operates at 70-80°C, therefore providing a higher kW output through a standard indirect coil than a heat pump, which generally operates at 55°C. As the heat pump is a lower temperature heat source, it is essential to maximise the heat transfer to the water held within the cylinder.

Many consulting engineers and even heat pump manufacturers recommend 3m2 surface area of the heating coil. Measuring a heat exchanger performance purely by its surface area is inaccurate, especially when comparing coil types such as a plain, copper integron (finned) or stainless steel corrugated heat exchanger.

The internal coil design in a heat pump cylinder differs from an indirect or solar cylinder, with most heat pump cylinders incorporating either a double or triple pass coil. What is critical is that the purpose designed heat exchanger provides optimised heat exchange, low pressure drop and allows higher flow rates to allow to heat pump to perform to its most efficient.

The cylinder is fitted with an integral immersion heater and thermostat, which raises the stored water temperature to the 60°C during its pasteurisation cycle, in line with guidance from the Hot Water Association (HWA), avoiding the risk of Legionella. The thermostat brings the immersion heater on at a temperature just below the heat pump maximum and switches it off at between 60-65°C, maximising the direct energy extracted by the heat pump and ensuring it is safe to use at the tap.

The Benefits Of Using Copper
Copper and stainless steel are the two most common materials used in hot water cylinder manufacturing and their associated heat exchangers today. This is primarily due to their high levels of resistance to corrosion.

It is a well-known fact that between the two materials, copper provides a superior heat transfer. Due to its high thermal conductivity, it becomes more beneficial to use, where maximising lower grade heat supplied from most heat pump manufacturers.

During recent testing conducted to establish the performance of varying metals, it was noted that Copper had over nine times better Thermal conductivity at 233, compared to stainless which was only 26, showing why copper is the perfect choice for transferring heat.

Bespoke Manufacture Requirement
When designing new build properties, the house layout provides sufficient space for the heat pump and cylinder installation. Unfortunately, this is not necessarily the case when it comes to retrofit applications.

In the recent BEIS heat pump trial, where free heat pump installations were being offered to trials in the North East and South East of England, it was noted that in the North East, whilst over 4500 people in Newcastle applied for the trial, only 13 went ahead due to the retrofit work requirements. One of the major contributory factors in retrofit heat pump installation is the siting of the hot water cylinder, as many properties had previously removed the traditional cylinder and gas boiler in favour of combination boilers.

Being able to manufacture a bespoke cylinder to fit the available space is crucial to the project proceeding. Whilst there are many heat pump cylinders manufactured to standard configurations, there are a huge number of installations which require slimline or horizontal cylinders and having the flexibility to produce cylinders in a variety of diameters, heights or even with tailor made tapping positions, makes the installers job easier and allows the installation to go ahead.

Setting the Standard
The HWA recently introduced a specific heat pump standard to sit alongside EN 12897 (for boiler heated indirect cylinders) and to ensure that the UK carbon footprint reduction achievable by a transition to heat pump use will be maximised by the matching with correctly designed hot water cylinders.

Within the standard, there is guidance to the test criteria so that installers will be able to compare the wide array of available cylinders on a like for like basis. With heat pumps varying in the way they operate, it will ultimately allow installers to match the correct cylinder to the chosen heat pump.

For more information and more tips on the ECOflow range of standard and bespoke designed cylinders visit www.mcdonaldwaterstorage.com

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