Renewable technologies working together

Project Overview
When planning a major re-build of his current home, Rev. Iain Rennie wanted to “do his bit” towards reducing CO2 emissions, whilst keeping his utility bills to a minimum during his retirement. Viessmann specified an integrated system of renewable solar thermal, solar PV and heat pump technologies which now saves approximately three tonnes of CO2 annually.

An integrated solution
Previously, the heating and hot water in Rev Rennie’s 1940s, three-bedroom property, had been provided by an oil-fired boiler. As the re-build had made the dwelling substantially bigger, it required a completely new system to meet heating demands.

The company favoured a Viessmann ground source heat pump system due to its low operating costs and high coefficient of performance (CoP) and specified a three-phase Vitocal 300-G heat pump, to fit with the existing 3-phase power supply. The heat pump supplies warm water to under-floor heating systems throughout the property, whilst a Vitocell 200-litre buffer cylinder ensures there is always sufficient heat energy available to cope with any sudden drops in temperature.

At very low outdoor temperatures, the heat pump is supported by an auxiliary in-line electric heater. For the majority of the year, this also helps the heat pump operate at its peak output. It works hand-in-hand with weather compensated controls, which tailor the output of the heat pump to the outdoor temperature, reducing the waste of redundant heat and the associated cost to the homeowner.

The lowest-cost energy source
During solar gain, the integrated control system suppresses the heat pump and instead collects energy from Viessmann Vitosol 200-F solar panels on the roof. This ensures hot water is always generated by the source with the lowest-cost. At other times, or when demand is greater than can be satisfied by the solar system, the heat pump is brought on-line to top up the Vitocell 300-B 300 litre hot water cylinder which stores the home’s hot water.

More commonly found in commercial settings, hot water is immediately available on opening a tap, due to a DHW recirculation system. This provides a background circulation of hot water ‘behind’ each tap. The timing of this circulation is also managed & optimised by the Viessmann Vitotronic integrated control system, accessed via the 300-G heat pump.

Electricity is produced on-site by a combination of the initial 2.25 kWp Vitovolt system, which has since been augmented by a second array as more roof space was created by additional building work. The harvested energy is fed back to the heat pump to further reduce its cost of operation, or back into the mains grid at times of surplus.

Compatible products, significant results
The heating system is now harvesting 25,720 kWh of free energy from the ground and running at a seasonal performance of 2.25 COP. Solar PV panels are also generating around 3.3 kW of electricity over the two arrays, with the solar thermal (for hot water) and solar PV systems working together to reduce CO2 production by 3 tonnes per year.

Compared to an oil-fired boiler, the installed system is costing 45 percent less to run, the equivalent of £972 per year. Without any incentive payments, the installation costs would be recouped in around 20 years, but support from the domestic Renewable Heat Incentive could reduce this by half when introduced. The customer already receives Feed in Tariff incentive payments for the two Solar PV arrays, and the government’s Renewable Heat Premium Payment has already paid £1,250 towards the heat pump and £300 for the solar hot water installation.