Power-Roof® Systems & Integration

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Power-Roof – the roof that pays for itself

Power-Roof® can be a vital tool in delivering near net zero, and it opens the door to a wide range of other services that can be integrated into systems for the house: energy storage, energy management, comfort, temperature and air-filtration/freshness control, energy export and other grid operations, and support for EV charging.

All of these additions are optional, but each adds value to Power-Roof® and helps deliver Net Zero or better.

The operator of a Power-Roof® system has access to a wide range of beneficial operations linked to the house, many of which provide added value and occupant comfort.

The Power-Roof® cassette: PV generation, heat harvesting and 325 mm of insulation in one factory-built unit, giving a roof U-value of 0.1.

Energy storage

A battery, hot-water storage and a heat pump let the roof’s output be kept for when it is needed – at night, or when the sun is not shining.

Energy management

A single controller balances generation, storage, import and export. All the occupant does is set a comfortable temperature.

Comfort & clean air

A heat pump linked to mechanical ventilation with heat recovery (MVHR) keeps the house at the chosen temperature 24/7, with filtered, dust- and pollen-free air.

Grid & EV

Export when the grid is short of energy, import when it has a surplus, and charge – or draw from – electric vehicles with vehicle-to-grid.

Basic Power-Roof system: PV to inverter, house and DNO; warm water to hot-water cylinder and central heating

Basic system

The basic Power-Roof® system delivers electricity from its PV and warm or hot water from its heat-collection circuits.

The electricity is fed to an inverter and then to the DNO connection. Whenever the sun shines, the roof’s output replaces imported electricity and meets the house’s demand, with any surplus exported.

The warm/hot water from Power-Roof’s heat-harvesting system can be fed into the central-heating or hot-water boiler, where it raises the input temperature and saves on electricity and fuel (gas or oil). Over a year this can represent more than 25% of a house’s heating bill (see the Data Sheet for heat-recovery estimates).

This is the least-cost system, but it brings very significant short- and long-term regulatory, financial and comfort benefits.

Note: our prototype systems have raised the input water temperature to a combi boiler by more than 10°C, a significant saving. With our planned aluminium-extrusion heat collection, we expect the roof to provide most of a house’s hot-water requirement for much of the summer.

Additional system storage

A sensible addition to the basic system is storage: a battery, heat storage and a heat pump.

A battery unlocks several benefits. Power-Roof® can continue to supply the house when the sun is not shining or at night, and there is the opportunity to export electricity when the grid is short of energy (for instance in the early evening) and import it when the grid has excess generation (when the sun is shining and the wind is blowing during the day).

Most batteries today are lithium-ion and, over time, can represent a fire risk, so they tend to be sited outside the house. Sodium-ion batteries are beginning to become available and may be less expensive and less of a fire risk, while bulk options such as vanadium flow may be ideal where several homes share electricity storage.

The heat output from Power-Roof® can be multiplied by a water-source heat pump, raising the temperature of the input to the heating system so that electric-resistive or fossil-fuel heating is no longer required. The heat pump can be powered from the PV, the battery, or directly from the grid.

Storage enhances the whole system, although battery storage is more expensive than simple water storage. Other heat-storage technologies include phase-change materials, which release significant energy as they change from semi-solid to liquid (ice would be a candidate if it didn’t expand on freezing).

Notes: Battery sizing depends on the electricity produced by the roof and the cost of cheap off-peak electricity. In many cases a battery combined with Power-Roof® can reduce the need to buy electricity to just the coldest, darkest winter months. Care is also needed to control the input temperature to the heat pump, as excess temperatures can damage it: sensing and control are required to recirculate cool heat-pump output back into its input and, at an optimum temperature, to bypass the heat pump altogether.

Power-Roof with battery, heat pump, dual-coil cylinder and hot-water storage

Power-Roof with battery, EV charger, heat pump, heating system and MVHR

Other system enhancements

Once Power-Roof® has provided sufficient generation to run the house, the next potential output is EV charging. With sufficient storage, and potentially vehicle-to-grid (V2G) on the vehicle, the whole system is enhanced.

The other major enhancement is Mechanical Ventilation with Heat Recovery (MVHR), linked to the heat pump. A heat pump can extract heat from the air entering the house through the MVHR, and the MVHR can provide a steady stream of input air 24/7 at the desired living temperature. Given the wide variability of external conditions, the control system needs significant thought. Because this air is also filtered and dust-free, the addition is a very healthy option – and because its power comes from Power-Roof® generation, the batteries and any heat storage, once installed its operation is effectively free.

If too much heat is being collected, Power-Roof® can be operated in reverse to dump heat to the atmosphere, as well as it being removed by the MVHR.

With the generation potential of Power-Roof®, we feel the more that can be done with its multiple sources of energy the better: these enhancements are essentially a one-off addition and require minimal servicing. One of the development areas for Power-Roof® will be to design and prototype the heat-pump/MVHR unit and its control system. Based on the cost of other individual devices, we believe such a unit could be made cost-effectively.

Note: an MVHR uses a filter to keep the air supplied to the house free of dust and pollen. Filters normally need regular replacement; research is needed into ways an incoming-air filter could shed its dust and pollen into the outlet side of the device.

External devices

External battery. Not shown in the diagrams, but for a group of houses – such as a social-housing or other development – there is a case for working with the DNO to site a large-scale battery that balances electrical use across the whole group. This is very likely to bring cost benefits of scale, and it also benefits the DNO, which can manage its energy flows better and avoid both local and upstream reinforcement. With clever switching, the whole area could also be isolated from the wider power system.

EVs with V2G storage. EV charging can clearly be an integral part of the Power-Roof® system. Many EVs now coming onto the market allow their batteries to be used as a source of power for the grid – or, in the case of a Power-Roof® system, for the building in which it is installed. The Power-Roof® controller could make use of EVs with V2G storage.

System diagrams

How the components of a fully equipped Power-Roof® installation fit together

Full Power-Roof® system

List of main components

Thin-film PV on the Power-Roof® cassette – for a full roof this may generate 15 kWp
Heat-harvesting collector, likely to be sections of aluminium extrusion
Power-Roof® insulation – 325 mm of Knauf DriTherm 32
Inverter, converting the thin-film panels’ DC output to AC, sized to the panels supplied
Smart meter – a requirement of the Distribution Network Operator (DNO)
Outputs to heating: radiators or underfloor heating as required
Circulation pump for the heat-recovery system

Air inputs and outputs for the MVHR system
Large dual-coil water cylinder holding significant thermal mass and isolating the cooling circuit
Water-source heat pump taking output from the dual cylinder, suitably blended to avoid excess temperature input to the heating system, heat pump or heat store
Heat store, which can also take excess generation via a PV diverter
Mechanical ventilation and heat-recovery (MVHR) unit
Blending system and anti-scald valve

The flow diagram shows the main elements of a fully equipped Power-Roof® system. We have taken care to avoid overloading the heat pump by feeding in water at too high a temperature; the circuits that bypass the heat pump, or blend the incoming liquid with cooled post-heat-pump liquid, are shown in the next diagram.

We consider that the optimum result for a new-build property will be where some heating is provided by underfloor heating to the ground floor, but general climate control is managed by the MVHR system – set up so that, as well as warming the air distributed into the house, it can also cool it. The diagram gives some idea of how best to manage this and how many variables need to be considered. It is probable that the house would be kept at a slight positive pressure, so that the only air taken in has been filtered.

Not shown is the case where localised area batteries are fed via the local distribution mains instead of batteries co-located with the in-house equipment. This possibility needs consideration and the cooperation of the local Distribution Network Operator; in many cases it may be the least costly battery solution.

Heat pump and MVHR assembly

Click the diagram to enlarge. Numbers refer to the functions listed below.

We have put a good deal of thought into how a water-source heat pump and MVHR system could be set up for Power-Roof®. We need to control the output of the MVHR to maintain a chosen temperature (with humidity control and dust filtration), while finding ways to avoid feeding fluid at too high a temperature into the heat pump. The main functions are:

Input and output to the Power-Roof® liquid-filled heat-recovery system
Controls and valves that circulate incoming and outgoing heat to the MVHR…
…bypass the heat pump if temperatures are too high, holding some heat in a heat store…
…and arrange a cool return to blend the heat-pump input and manage its temperature
The heat pump itself – likely to be similar to the Kensa Shoebox, as the power output required is not expected to be great, given the minimal heat losses from the house
Controlled inputs and outputs from the house heating and hot-water system

The system controller – the whole system needs a sophisticated controller to optimise performance (the green dotted lines show what it controls)
Temperature sensors in the house, controlling the output to the heating and hot-water systems
MVHR intake of clean air from the environment and return of stale air
Input and output of air to the house, with sensors to adjust temperature to achieve the required house temperature
Heat exchangers that take hot or cool water from the heat pump to heat or cool the MVHR intake – or take excess heat from the MVHR and feed it to the heat pump as necessary
The MVHR unit and its systems

This flow diagram shows how the majority of the variables in using Power-Roof® together with a heat pump and MVHR can be managed. As well as the basic MVHR function, hot or cooler water can be fed to heat exchangers to deliver the required input temperature to the house. The MVHR’s air filters ensure that the air entering the house is clear of dust and pollen; some work is required to develop strategies for making these filters self-cleaning – possibly by rotating the filter into the outflow of the MVHR and vibrating it to remove dust particles. Some research may be needed here.

Construction requirements

New build

For new-build houses implementing the Power-Roof® system, the builder or contractor will need to connect to the hot-water system and, if underfloor heating is installed, to the heating system. The MVHR needs a carefully designed and installed ventilation system to deliver clean air at the required temperature and humidity and to collect the stale air. The specification of the ducting to eliminate noise will be important, as will the placement of vents to ensure an even distribution of fresh warm/cool air in the house. Ducting may need to be flexible, with minimal sharp corners.

Additional items include dual-coil hot-water cylinders to keep the liquid circulating through Power-Roof® separate from all other water circuits. There is likely to be a hot-water cylinder downstream of the heat pump, sized to store sufficient hot water for an extended period.

Retrofit

For retrofit, care must be taken that remedial internal work is kept to a minimum. This means that MVHR and ducted fresh-air systems may be limited to the floors immediately below the roof space. These systems are also likely to be bespoke and should be approached with care to avoid unnecessary cost.

Energy storage

A Power-Roof® system needs energy storage to level out the energy available from the roof and balance it with what the home needs – especially during periods of low external temperature and shorter daylight (winter).

Day to day heat storage

A decently sized dual-coil cylinder will cover 24–48 hour fluctuations in demand, but must be sized accordingly.

Long term heat storage

Managing the November-to-February period might need a system capable of retaining significant amounts of summer energy, so there is sufficient stored heat to make up the balance from mid-November to the end of February (this needs careful cost-benefit evaluation).

There needs to be an estimate of any shortfall in input heat to the system, noting that drawing ambient energy off Power-Roof® will be possible at temperatures down to 5°C (lower temperatures risk ice build-up and a deterioration in PV generation). The shortfall may only be around 20 days, but a winter similar to 1963 – almost continual freezing conditions and lying snow from January to March – may need consideration.

For new build, there could be a case for local heat stores: a large, highly insulated underground storage tank that tops up heat for short periods in winter, charged in late summer and drawn on during cold days. Since a new-build house meeting new social-housing regulations will have very low energy losses, sizing such a tank and its insulation will need detailed calculations.

Battery storage

Electrical energy storage raises several considerations, since the electrical system’s connection to the local distribution grid – operated by the Distribution Network Operator (DNO) – is likely to require the least possible load, both as output to the grid and as input from it, and the DNO is likely to be more amenable to connection where these are minimised. There is also the case where the DNO is itself involved in the battery storage. Key considerations:

Siting and type of battery. Few commercial batteries are recommended for siting inside the house: lithium-ion is frowned on by insurers because, in the rare event of fire, it cannot easily be extinguished and can burn a house down. Sodium-ion is less prone to ignition, but batteries generally still need siting externally. It may be sensible to work with the DNO to site one large battery – perhaps vanadium flow – remotely, supporting several neighbouring properties.
Sizing the batteries. The system needs to run electrically 24/7 with minimal necessary import from the grid for at least 24 hours, taking into account the heat pump, MVHR and, where fitted, EV charging. A local area battery holding up to a few MWh may prove cost-effective for several properties.
Import/export policy. There may need to be a policy to import electricity from the grid when the grid needs to dump electricity (potentially a paid service to the importer), and to export stored electricity to help cover temporary shortages, adding further value to the system.
Major requirements: store all the energy a house needs for 24/7, all year round, on a daily basis; minimise the load on the local grid in both import and export; and ensure the overall system is self-sustaining, mainly or wholly from renewables.

System control

The key to a satisfactory internal house climate with the Power-Roof® system will be its control.

Ideally, all the occupant should need to do is set their most comfortable temperature; the Power-Roof® controller will then manage its systems to deliver exactly the right amount of heat (or cooling) to the heating and ventilation systems. The system should be fully automatic and may well use AI to manage all the variables – including the best times to sell energy into the grid, when to download energy that is virtually free, and forecasting future weather so it can pre-emptively store energy and keep the house at the required temperature.

Humidity may also be managed, both by removing humidity from incoming air and by misting the air directed into the house. This function will be aimed at ensuring that no potential mould conditions can arise in any house.

Where a local area battery is used, the system control will need to match the import/export of electrical power from the house to the import/export of the battery, in concert with the control systems of the other houses connected to it. The smart meter is likely to need to track these flows, so that use by one house at the expense of another is credited within the cost and income from their connection to the grid.

It is also very likely that the system control will communicate with the grid and work to help balance overall demand – which in many cases will have a value over and above the energy flow. A major project as Power-Roof® becomes widespread is to optimise these connections and help accelerate the national drive towards net zero.

Stephen Pattenden, ACTL