Energy storage is a fundamental requirement as we transition to renewable energy sources. Thermal energy storage is very effective, has high energy density and very low cost.

2022 Top Article - Solar PV and Thermal Energy Storage

Q&A with Mitch Hodges, Co-Founder & CEO | GreenPWR

Tell us about yourself and your role with GreenPWR.

Mitch Hodges- cofounder and CEO of GreenPWR.  I am an electrical engineer with prior experience in mobile devices, IoT devices, smart lighting systems, and now into renewable energy devices. 

GreenPWR started after I left my prior job in the mobile device space and studied for my masters in sustainable and renewable energy at the University of Nottingham.  During my studies, I met the Hockerton Housing Project team (currently one of our cofounders in GreenPWR) and began work on a renewable energy storage system using PV for hot water.  After that short project timeframe of 3 months, we had a working prototype.  It was clear this was a feasible product for the market and modernised renewable hot water production, so we proceeded forward with bringing this to market with our 2 additional cofounders.


Where are we with Solar Thermal?  How has technology advanced hot water production via PV over the last few years.

Innovations in solar thermal systems have reached a plateau.  Simple thermosyphon systems with a tank on the roof exist, although the installation practicality is limited.  Any temperate climate where freezing is possible will require the more complex closed loop system which has plumbing from the roof through the home, a specialized hot water tank with a heat exchanger, and a pump.  These components have possible failure cases which are catastrophic to the solar thermal system.  With any plumbing system, leaks are possible.  Overheating is another concern in the event there is little water usage or if a pump were to fail.

PV has seen decreasing prices over the years.  This has allowed systems based on PV to become more viable.  Some companies have begun to create products to support this special use case, though this is still in its infancy.


What are the benefits of generating hot water via PV?

Hot water via PV brings several practical advantages over traditional solar thermal systems.  Installation is much simpler.  Wiring is used to transfer energy from the PV array to the inverter instead of a thermal transfer fluid via plumbing. 

With the PV system there is no risk to overheating as the energy is collected via electricity.  The overall system is optimized for installation by using off-the-shelf components.  An existing immersion heater electric water tank, PV panels, a mixing value, a rapid shutdown device and a SunHopper-D are the required items.  This increases installation speed dramatically and keeps costs low.

PV panels have decreased in cost over the years.  This allows the SunHopper-D system to beat the cost of solar thermal systems and be much simpler to install, making this a modernized method of generating domestic hot water via solar.

Additional rooftop space is indeed required when using PV instead of solar thermal as the collector efficiency is lower.  Although this is the case, the benefits of the system still outweigh this downside.


Tell us about your product the SunHopper-D and how thermal energy storage is accomplished with the hot water tank within the home

Energy storage is a fundamental requirement as we transition to renewable energy sources.  Thermal energy storage is very effective, has high energy density and very low cost.  By looking at the hot water heater as an energy storage device, we see nearly all homes have this energy storage medium to utilise. 

The SunHopper-D uses the hot water tank as an energy storage device.  Energy is not recovered as electricity, but as useful heat for domestic hot water.  The supported panel arrangement is three to six 60 or 72 cell panels, however typical installations will require 4-6 PV panels.  As a rough rule of thumb averaged over the year, one PV panel will produce between 7-12 gallons of hot water per day.


What is the SunHopper algorithm?  What makes it beneficial and how is it the key to pairing solar with a low cost thermal storage for later use?

Using hot water as an energy storage medium is not the same as a battery.  The home owner expects water to be hot when required.  Our patent pending SunHopper algorithm monitors several aspects to ensure minimal energy consumption is used to ensure hot water is present.  These include local weather forecasts, past water usage, anticipated water usage, shade conditions of the installation, time of use electricity rates, and the state of the thermostats inside the water tank.  By utilizing this array of inputs, the SunHopper algorithm is able to keep the balance between energy savings and hot water availability.


Where do you see the SunHopper being installed?  Share with us some possibilities and benefits.

The SunHopper is designed to support as many installations as possible.  This can be a complete off-grid setup where only a dedicated PV array is provided.  The system can also be installed with a grid connection which can be used as a backup energy source, or have a fossil fuel backup using either natural gas or propane.  In the case of fossil fuel backup, this would entail a 2-tank installation where the SunHopper can preheat water for the fossil fuel backup stage.

In the total off-grid case, the SunHopper will store as much energy into the hot water tank as possible.  The AC input can be connected to a battery backup inverter if required to assist in supplementing energy from a larger array, though this is optional.  This can work for a small cabin, or for sinks in remote locations (e.g. bathrooms on a hiking trail).

The more common installations will require some backup energy source to overcome weather variation.  The average home would be able to reduce energy consumption by 60% to 80% on average for hot water production.  The average single family home uses 17% of its energy consumption to produce domestic hot water¹, hence this is the second largest energy consumer in the home which the SunHopper-D can show large energy savings.


Why Phone app functionality for monitoring and why did you feel it so important to offer monitoring?

Our phone app allows remote monitoring, performance monitoring, and system control in a convenient manor.  Our phone app helps show the energy produced by PV and energy consumed by the grid if using an electrical backup. 

Control is also provided to optimise hot water production.  Our algorithm has several settings, being our normal mode, eco mode, high demand mode, and vacation mode.  Normal mode balances energy efficiency with hot water availability using previous learned usage patterns.  Eco mode is more aggressive for saving energy, though the hot water supply may run short if deviations in usage exist.  High demand mode can be controlled when required- for instance if a family member were visiting and additional hot water is required.  This will consume more backup energy, though.  Vacation mode disables the backup energy into the tank and only runs on the PV panels to reduce energy consumption.  These settings help the user select the best plan and optimise the balance between energy savings and hot water availability.


Tell us about your future vision with energy storage and the path to future products.

GreenPWR is focused on the need to pair renewables with energy storage systems.  Our next product will also use the domestic hot water tank but will use an existing PV installation and divert energy into this in a smart fashion with our SunHopper algorithm.  This enables a clean installation for those who already have solar to add energy storage without a very large battery.  Current products do exist in Europe which complete this task, though our advantage with our algorithm will push the envelope.

Beyond this we are investigating what other mediums within the home can be exploited as energy storage devices along with demand side management to assist in levelling the energy consumption in the home.  These next concepts expand and fill the current void in connecting energy consuming appliances together with energy producing devices.  These are exciting and interesting concepts we are working through to maximise self consumption.




  1. U.S. Energy Information Administration. Use of energy explained. June 23, 2021.
The content & opinions in this article are the author’s and do not necessarily represent the views of AltEnergyMag

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