The iHP delivers hot water 10x faster and lasts 4x longer than alternative systems on the market.
The iHP has been made possible by Stratos, Mixergy’s novel heat pump technology which stratifies energy within the cylinder, to provide the best combination of quick reheat times and high efficiency.
This article delves into some of the detail around how heat pump cylinders work, but before going down that road, it’s worth asking…
A heat pump cylinder is simply a cylinder with a small heat pump on top. It’s designed to provide all the hot water needed in the home for a fraction of the energy, normally consumed by an equivalent gas or electric system.
Let’s be honest, heat pumps have their challenges, but these are a consequence of our traditional approach to space heating, rather than a fundamental issue with heat pumps themselves. In the UK we have become used to powerful on-demand boilers, delivering high-temperature heating from small radiators fed by even smaller central heating pipes. These traditional wet heating systems can’t cope with the high flow rates and low temperatures necessary for a heat pump to work efficiently.
Consequently, the typical cost of installing a new heat pump ranges wildly from £15,000 to £30,000 depending on the amount of new plumbing, electrics and redecorating involved. This high capital cost, combined with the shortage of skilled trades throughout the UK, is the reason that a ‘Fabric First’ approach to net zero is often advocated.
Instead of diving straight in with a new monobloc heat pump install, ‘Fabric First’ means lagging the loft, filling the wall cavities, excluding the drafts and upgrading the glazing to reduce the heating demand as a priority. This gives more immediate incremental results whilst putting the home on a pathway towards the point where a future heat pump install could become viable as the heat load reduces and costs come down. But is there a bigger initial step between incremental ‘Fabric First’ measures on the one hand versus a whole house retrofit project on the other? The answer is yes!
Heat pump cylinders are already familiar to those living throughout warmer regions, such as Southern Europe, where space heating and hot water are often kept separate from one another. Now, heat pump cylinders are gaining significant traction in the UK as a retrofit measure which can quickly uplift the energy performance of a home, but with less disruption.
Heat pump cylinders bring a considerable reduction in hot water energy consumption, which accounts for anything up to 20% of total household energy consumption. If a landlord or householder wishes to go further on space heating, there is a range of electric solutions depending on how extensive the treatment of the fabric has been. This ranges from anything from a heat-only heat pump, to lower-cost direct or infrared electric solutions, where very high levels of thermal performance can be achieved.
To answer this question, we first need to talk about how a regular heat pump cylinder works. The following diagram shows a cross section through a conventional system:
There are two core parts, a conventional (usually pressurised) hot water cylinder and a small heat pump fixed on top within a ‘head unit’. Typically, ducting from outside is fitted straight into the head unit which includes a fan to force air over a heat exchanger, called an evaporator.
The evaporator is very similar in construction to a car radiator. It has lots of convoluted copper pipework and aluminium fins to provide high levels of heat transfer from the incoming air to the refrigerant circuit. When the heat pump is running, the compressor takes low-pressure refrigerant from the evaporator and boosts it into the condenser jacket which is wrapped around the outside of the cylinder.
This boost in pressure causes the refrigerant to get hot (imagine the end of a bicycle pump when you’re inflating a tyre). The hot pipework around the cylinder heats the body which in turn transfers heat into the stored water. As the hot refrigerant comes back into the head unit, it is allowed to escape from an expansion valve into the evaporator which is operating at low pressure.
If you have ever seen ice collecting on a propane bottle outside a burger van, you’ve witnessed the same process that unfolds within the evaporator of a heat pump. It’s the sudden drop in temperature due to the changing state of liquid refrigerant into gas as it experiences a rapid drop in pressure. The resulting drop in temperature means the refrigerant gas flowing through the evaporator is now colder than the air that is being pulled through the head unit by the fan.
Being colder than the flowing air, the refrigerant pulls heat in through the copper tubing and aluminium fins within the evaporator. This elevates the temperature of the cold refrigerant before going back into the compressor. This warmer refrigerant now possesses some thermal energy which has been taken from the outdoor air and is now returning back outside at a slightly lower temperature.
The warmer refrigerant is about to get much hotter again as it goes back into the compressor for another journey around the refrigerant cycle. The magic of the refrigeration cycle lies in its ‘Coefficient of Performance’ (COP).
Typically, we transfer around 3x more heat from air to water during the refrigeration cycle in comparison to the electrical power consumed by the compressor. Think of this as a ‘conveyor belt’ for heat. Instead of the heating element in a kettle, imagine a small motor moving a belt with big boxes of heat from outside air into the cylinder. But despite their considerable efficiency, there are some serious shortcomings with conventional heat pump cylinders…
To fit a heat pump on top of a cylinder you must make it small. Consequently, the power output is low (anywhere between 1.5kW and 3kW). Think of a regular kettle which is about 3kW in power. Imagine heating a bath with a kettle! Heat pump cylinders on the market today take anywhere between 6 and 10 hours to get hot. Such glacially slow heating means that people opt for bigger cylinders to avoid running out of hot water which makes them even slower.
The heat pump head unit and hot water cylinder in existing products are inextricably linked by the pipework of refrigerant wrapped around the cylinder. You do not want refrigerant to ever escape from the system because it may be explosive or have a high global warming potential. As a result, all the connections between the refrigerant lines, the head unit and the cylinder are brazed solid, meaning that the cylinder and head unit can never be separated.
If there is a problem with the heat pump, the whole product (head unit and cylinder), must be scrapped. Given the punishing life of a typical heat pump, manufacturers are only willing to provide a 5-year warranty on integrated heat pump cylinders compared to the 25-year lifetime of a stainless-steel cylinder.
Conventional heat pump cylinders can weigh in excess of 100kg and the head unit cannot be separated from the cylinder (for the reasons outlined above). This makes the product difficult to install and navigate up flights of stairs. Distribution also becomes problematic due to the increased height of the product meaning specialist couriers are often required.
Conventional hot water cylinder manufacturers are not equipped to handle refrigerant and complex assemblies like heat pumps create a significant burden on their facilities. Wrapping a refrigerant coil around a cylinder and ensuring good thermal contact is a specialist and labour-intensive process which adds considerable expense.
In developing the Mixergy iHP, we decided to take a fundamentally different approach with the following objectives in mind:
This is achieved by our modularised Stratos heat pump head unit. The Stratos instantaneously heats hot water which is drawn from the bottom of the cylinder and returns it back to the top to achieve very quick reheat times. Alternatively, a flow path back to the bottom of the cylinder allows the system to achieve the highest possible COP on a full reheat cycle.
This arrangement allows us to achieve a useful temperature 10x more quickly so that useable hot water becomes available in 30 minutes, compared to 10 hours. This is achieved whilst delivering savings of up to £350/annum and reducing hot water energy consumption by two-thirds or more.
The following diagrams illustrate how the system works when compared to a conventional heat pump cylinder:
Initially, both cylinders start out cold with water at 10 degrees Celsius. Within 30 minutes, the Mixergy iHP has developed a useable layer of hot water at 50 degrees Celsius, whilst the conventional hot water cylinder has only been able to warm up to 15 degrees Celsius.
This fast reheat speed means that a smaller Mixergy cylinder can be specified so that after 3 hours the iHP has developed a full cylinder of hot water, whereas the conventional cylinder has only just reached near a tepid 40 degrees Celsius or so.
This fast reheat process from Mixergy allows for a quick boost with more than twice the efficiency of a direct electric heating element. On the other hand, the system can opt for the highest system efficiency (COP ~3.5+) through whole cylinder heating via the boost divert valve.
With a modularised approach to heating, the Stratos can be decoupled from the hot water cylinder, this makes the product simpler to manufacture and distribute on the one hand and considerably easier to service on the other. As a result, we offer an extended warranty of 25 years for the cylinder with an extended warranty on the head unit being made possible through its improved serviceability.
Combining Mixergy’s ‘heat what you need’ approach to hot water storage alongside the capability of our state of charge sensor, connectivity and machine learning, the iHP is future-proofed to make the most of renewable energy whether it comes from rooftop solar PV or flexible tariffs connected to offshore wind.
The team at Mixergy were honoured to receive the Best Brand New Product award from Housebuilder. It has taken two years of hard work within the technical team to get to this point and we’re extremely excited to be launching this product at such a critical time on the journey to net zero.