Scottish Solar Home: Adding value is not about adding subsidy

20.11.2015
Both the solar thermal collectors and the PV panels are from the same supplier, Viridian Solar, and are designed to look nearly exactly the same. (Photo: Solar Kingdom Ltd)
Both the solar thermal collectors and the PV panels are from the same supplier, Viridian Solar, and are designed to look nearly exactly the same. (Photo: Solar Kingdom Ltd)

A Scottish ultra-low carbon home won a prestigious Solar Award using a very uncommon solar installation. Ignoring some subsidies schemes, both PV and solar thermal panels are arranged in a quite unusual east-west-configuration.

Trying to convert an existing building into a low carbon building is always challenging. You do not only have to cope with all the typical and atypical technical difficulties that old buildings bring with them. You also have to deal with a lot of government regulations and complicated subsidy systems.

The now awarded building inEdinburgh has a very interesting approach to deal with all this obstacles. Additionally the also installed PV-system is built in the same configuration. However, you already can find quite a few east-west PV systems, especially in countries where self-consumption is getting increasingly important.

But let’s start at the beginning. The renovation of the building which is owned by Rob Goodburn, owner of the architecture company Goodburn Baillie, started with external and internal insulation works and installing triple glazed windows. Rather than going for the best U-values, the main objective in doing this was to maximise the decrement delay in order to lower the peak load on building service systems.

Heat storage is the heart of the system

Like in many other buildings that are using renewable energies for heating, the heat storage is the heart of the system. In this case it is a 600 litre heat storage with an internal heat exchanger for production of hot tap water. The storage tank is fed by three different heat sources: A batch log stove, solar thermal collectors and – only as backup – a 12 kW gas boiler. The local Edinburgh solar specialists Solar Kingdom aided evaluation and planning to optimise the integrated use of the technologies. Solar Kingdom were further involved with the installation and commissioning.

Since the roof is not faced southward the solar installations had to be faced to the east or west. Thanks to the heat storage a solar thermal system alone could have been built easily only on one side of the roof. However, that is very different for the PV system. Electrical storage is still much more expansive. However, this differs from market to market. Germany, for instance, has a very active market for electrical storage solutions since it has some attractive subsidies (which unfortunately could be stopped in the near future).

Flatter production of electricity

However, since electrical storage was not an economic solution the 5 kW PV system was built in an east-west configuration. That means that the production of electricity is somewhat smaller than that of a standard southward installation. On the other side the production is flatter with a lower peak during mid-day but with a larger production in the morning and evening. This configuration is not optimal if you want to feed as much electricity as possible into the grid.

But it has advantages if you want to use as much electricity as possible for yourself, as most people are not at home during the mid-day peak. The PV system is expected to produce around 3,600 kWh of electricity per annum. How much of that will be fed into the grid and how much can be used by the family is not yet clear.

Each collector with a solar circuit of its own

The configuration of the solar thermal collectors followed that of the PV panels. On every side of the roof one 3.1 m² solar collector was installed. This means that there are two solar circuits with each using an own solar pump. Both the solar thermal collectors and the PV panels are from the same supplier, Viridian Solar, and are designed to look nearly exactly the same, to ensure an appealing appearance.

Configuration of the solar thermal system  (Graphic: Solar Kingdom)

Using the Heat Incentive would have been a mistake

The solar thermal system was built outside the framework of the Renewable Heat Incentive (dRHI), which is not that uncommon in Britain (see also Sun & Wind Energy issue 5/2015, page 26). In this case the problem was that the solar thermal heat is not only used to produce domestic hot water but also for space heating. The dRHI is only available for hot water systems.

Although the system only has a collector area of little more than 6 m² it often produces more heat than is used for hot water. Also under certain conditions heat is produced at a temperature level that is not high enough for tap water but usable for space heating which only requires 40 °C.

So using the solar system also for space heating is obviously the best thing to do to avoid burning wood or natural gas. But at the moment you have to work outside the subsidy system to get the best heating system.

Jan Gesthuizen

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