Simple energy savings from smart hot water tanks

Mixergy tank line-up

By heating water from the top-down, the Mixergy tank allows people to reduce energy use and manage the natural uncertainty from renewable supply. Franca Davenport speaks to Co-Founder and CEO Pete Armstrong to find out more about this seemingly simple technology for Undaunted’s Climate Innovations Up-Close blog series.

According to the Energy Savings Trust, household emissions from heating and hot water must reduce by 95 per cent to reach the 2050 net zero targets and, with the current rise in energy costs, the incentive to do this is even greater.

By taking a novel yet intuitive approach to heating water, the Mixergy tank offers an opportunity for households to reduce their energy use by heating water more efficiently and enabling those households with renewable sources to maximise their low-carbon supply.

Mixergy logo

Stratifying water by temperature

Conventional hot water tanks take an all or nothing approach to heating and use a coil that sits at the bottom of the tank. The cleverly engineered Mixergy tank takes advantage of the simple fact that hot water is less dense and rises and cold water is more dense and sinks, also known as thermal stratification. By placing its heating element at the top, Mixergy have designed a tank that can selectively heat water so that the hot water effectively floats on top of the cold. This enables households to heat just the amount of hot water they need, which can save up to 20 per cent on hot water bills. By heating smaller volumes, the water is also heated in a shorter time – about two to five times faster than traditional tanks. 

These smart tanks have also addressed another problem of conventional heating where the process of drawing off hot water causes incoming cold to dilute the remaining hot. Mixergy have created an inlet diffuser to minimise the mixing of different temperatures inside the tank so it can retain about 30 per cent more usable water.

Low-hanging fruit of heating water

Co-founders Pete Armstrong and Ren Kang came up with the idea for Mixergy when they were completing their PhDs in engineering at Oxford University. They were investigating different energy storage technologies and realised the potential savings that could be made through relatively simple adjustments to the way people store heated water.

Pete Armstrong
Pete Armstrong

“It’s the low-hanging fruit of energy efficiency in households,”  says Armstrong. “Rather than heating the whole tank of water like a big kettle you can turn it on its head and heat from the top-down.”

They started developing the technology and in 2015 installed four Mixergy tanks in a trial with GreenSquare with social housing tenants. The team then shifted their focus to the tank’s potential to enable households to optimise energy generated by renewable sources, making the most of its volumetric heating approach which enables solar energy supply to match the demand for heating water.

In 2017 Mixergy joined the Imperial Climate Accelerator. They began to grow the team and embarked on a project to install 500 smart hot water tanks that would deliver a load balancing service to the National Grid to help store excess power during low demand periods. “The Imperial accelerator always provided a friendly sounding board,” recalls Armstrong. “It was a great catalyst for our development, both financially and emotionally.”

“For us it was critical to make the technology agnostic…whether it’s a heat pump, a PV array on a roof, or an existing gas boiler that is being upgraded to a hydrogen boiler. Whatever the source, we wanted our tank to get the best out of the system.”

Pete Armstrong, Co-founder and CEO, Mixergy

Keeping options open

In 2018 the technology featured on the YouTube channel Fully Charged, an opportunity that came about from what Armstrong describes as a ‘guerrilla marketing’ technique where he approached presenter Robert Lewellyn at a book signing in his local shop. With nearly a million subscribers to the channel, this produced an influx of interest in the Mixergy tank but, alongside this, came a realisation that, even with this level of public interest, the company needed to have installers on board for the technology to reach the customers. “We soon discovered that installers aren’t a monolithic community,” says Armstrong.  “And this taught us the importance of segmenting your audience so you can target them effectively.”

Mixergy tank

In 2021 Mixergy received accreditation from the Energy Saving Trust and in 2020 the tanks were launched nationwide through British Gas. With the arrival of the pandemic that year it meant many installers had to purchase online rather than through merchants, which was an enabling factor for the adoption of Mixergy, making the supply chain more direct. Increasingly Mixergy is becoming a name that is known both by householders and by the trade.

With their continuing mission to enable homeowners to heat their water effectively from their choice of renewable source the company has designed a solution for heat pumps. This uses an externally mounted plate heat exchanger to transfer heat from the pump to the water inside the tank. By transferring heat over the small surface area, the exchanger is more efficient than the large internal coils which are used on conventional heat pump tanks.

Home energy management

To help manage the different elements of its hardware, Mixergy developed an app that enables users to be in control of their tank 24/7. This management system will also be central to the products they are bringing into the Mixergy family such as the heat pump exchanger and future plans for an EV charger.

And with its sights set on innovating for a range of renewable energy sources, Mixergy has secured a £9.2 million investment in 2023 to support a new R&D centre to further develop their home energy and heat storage technology, alongside their international expansion. “We want to make our technology scalable,” says Armstrong. “But we also want to be able to move quickly enough to respond to the market. By keeping our options open in terms of energy sources we hope to bring our technology to as many people as soon as possible.”

Climate Solutions Up-Close

Take a deeper-dive into innovative climate technology by exploring other posts in Undaunted’s Climate Solutions Up-Close blog series.

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