The installation was undertaken with grant assistance from Sustainable Energy Ireland (SEI), who are contributing appx EUR1,900 toward the cost. For my particular installation, the specification was as follows;

The amount of grant given is based on the aperture area of the tubes: the remainder of the system does not alter the grantable amount. Also, the installer must be registered under the scheme and approval sought before any work begins. I received the grant paperwork in advance from the installer and then posted it to SEI. Once I received an offer letter by return, I proceeded to place the order with my installer. 

Cylinder Placement:

Very few homes I know have a hot press large enough to handle a 750 litre buffer tank, so usually the buffer tank will need to be sited elsewhere. Also, this size tank is probably at the limit for fitting through a standard door, and even then it took a lot of manoeuvring and three people to get it through! So you need to bear this in mind if opting for a system of this size. In the picture below, the buffer has been placed in the utility room and all connections to it come from the boiler room located to the right, with all pipes to/from solar panels and DHW going directly through the roof space.

The pipes for the DHW are routed back to the airing cupboard. Here, the original 130litre tank is removed and the pipes that led to it are capped and closed. The pipes coming from the new buffer tank are then connected to the original pipes that flowed out from the old tank, thus completing the hot water circuit. The only addition is a small uninsulated copper pipe placed in the airing cupboard to provide a trickle of background heat for airing of clothes and so forth. This could be replaced in time by a small radiator or a larger pipe run, if more heat was required.

The mixing valve, shown in the inset picture, bottom right, allows the temperature of the hot water to be adjusted, by mixing the hot water from the buffer with cold water before being distributed to the taps. This ensures the water does not get too hot at the taps, especially on days with maximum solar gain. The only problem with the mixing tap is that it works best when the hot water is at a constant temperature, so that the mixing of hot and cold water results in an even temperature at the tap. In reality, this may need to be adjusted up or down depending on the tank temperature.

Evacuated Tube Placement:

Having installed photo voltaic (pv) panels myself, I can understand much of what was done to install the evacuated tube framework. First, the tiles are exposed and then battens are nailed in place along the rafters. This allows the roof hooks to be installed at the correct points, and does not have to rely on hooks being spaced along rafters only. Once the hooks are fastened to the battens, a groove is cut in each tile so that it can sit flat even with the roof hook extruding. This ensures that there is no possibility of rain blowback. (Incidentally, this was not a concern with the self made hooks I used for the PV panels as the hooks were much thinner- for the commercial framework in use here, the hooks are at least 5mm thick).

With the hooks in place, the tiles are pulled down again to makes the roof watertight. Now the mounting manifold is secured to the hooks at the top and bottom, and finally each tube is fitted and connected to the manifold. Tubes can be rotated appx 20 degrees to account for orientation to the east or west, so here they were offset a little due to the East-South-East orientation of the roof.

Finally the manifold is connected to the flow and return pipes from the buffer tank. The roof tile is drilled to allow the piping to come through and then bound with lead flashing. This is the only point the roof is actually bored through, but with the lead flashing running from the tile above and protruding higher than the hole, it is impossible for any rainwater to get through. It is also bonded with silicon and the pipes lagged with Armaflex to help with heat retention.

Solar Fluid versus Water:

There are differing systems on the market to transfer the heat from the tubes to the tank. Some systems use water, and while this undoubtedly makes it a simpler solution, care must be taken with such a system to ensure water is always being pumped in cold weather to reduce the risk of freezing. Other systems, like the one installed here, use a heat transfer fluid. This is a mixture of water and other ingredients, including anti-freeze, that is specially prepared for use in solar evacuated tube and flat panel applications. Whilst the anti-freeze element ensures that it does not need pumping during cold weather, the properties of the anti-freeze degrade over time, requiring a refilling of the collector fluid every 5 years or so.

Solar Controls and Tank Heating:

The solar controller than comes with the system (inset picture: buffer tank, top right) has one main job to do- which is to monitor the temperature of the water both at the base of the tank and at the evacuated tube manifold. If the temperature of the water at the collectors is 4-5°C higher than the tank, then the solar pump is activated and the hotter fluid is pumped down from the tubes and into the buffer tank coil. Then, by a process of heat transfer, the surrounding water in the buffer tank is heated. In turn, the water in the outer buffer tank now heats the water in the inner tank, giving us the hot water that will flow to our taps.

Importantly, the solar pump does not activate if the tank temperature is higher than the manifold temperature (e.g. at night), otherwise we would cool down the water a little which would be wasteful. And a bit silly.

Boiler Connections:

The oil boiler which initially fed the radiators is connected directly to the buffer tank, via a dedicated 2nd coil. This provides us with the top up for the central heating support that will be needed as winter draws in. When the heating is switched on, if the temperature of the buffer tank is 60°C, then the boiler stat will keep the boiler off. Once the water from the buffer tank is circulated through the radiators, the return temperature will begin to drop, and at that point the boiler senses the drop and kicks in. This is exactly the same principle that would be applied if running with a different heat source such as a wood pellet boiler.

Total SUN Power:

If you recall from my renewable electricity project, I have the option of swapping over circuits from the GRID to my renewable electricity which I generate myself. The final step was to test the power requirements of the solar controller and pump, which came in at 2watts and 85watts respectively. No problem for my electricity supply to handle. So all I did was to connect up the solar controller and pumps to the utility room sockets, and hit my changeover switch, and voila- the system was running Off Grid.

Energy Savings:

The savings are calculated on a two-fold basis:

1) heating of DHW will now be controlled by solar, not by the oil boiler. Even on winter days, when the temperature of the buffer tank may only reach 30°C, that may still be sufficient for hot water, baths, and showers. During the summer months, when there is no space heating requirement, the boiler can hopefully remain turned off.

2) space heating requirements are reduced because initially, if the buffer tank is already at the right temperature, then the oil boiler will not start up. So we already get the benefit using this initial heat without having requested the boiler to turn on. Whenever it does finally turn on, the water in the buffer tank will never be really cold, so the boiler only has to heat up the water from a warm base, rather than heating from a cold base as it would have done in our previous system. Finally, using a buffer tank should also mean less frequent on/off boiler cycling, because the large mass of water will hold its temperature longer and ultimately use less oil than before.

The proof of this cannot be measured at this early stage, so I will be monitoring the temperatures closely over the coming months to see how well the system perform through an Irish winter. Then we can check the volume of oil we are using and draw some comparisons with our previous system.

Energy Update Jun 2008:

Having monitored our oil usage throughout the year, it was now time to to determine the energy savings. In order to ascertain an accurate figure I noted the dates when an oil fill took place and then calculated the number of days between fills. I then compared this to last year, prior to having the system installed.

Unfortunately, my calculations revealed that, on average, there was no discernible difference in our oil usage over the period since the evacuated tubes has been installed. I contacted the installer with my concerns, and they inspected the system. There was nothing inherently wrong with the set up, but I did receive a newer solar controller, which uses a pulsed motor action rather than an on/off action to slow down the fluid transfer. This may lead to some additional savings over the course of next year as the fluid remains in the tank longer to effect the heat transfer. They also advised that any outgoing hot water pipes coming from the buffer tank be lagged also, to further slow down any heat loss from the tank.

Of course getting a true figure is not an exact science, unless every variable was the same. I am unsure if last winter was more severe that previous ones- there's always the possibility that we simply used more oil, or perhaps that our oil boiler needs another service to restore the efficiency. 

There is no doubt that through the summer the advantages are obvious, but there remains an underlying concern that it is not doing "what it says on the tin" in terms of adding heating support during the winter months.

In the longer term, with a plan to move away from oil, we would have had to invest in the larger buffer tank anyway - for example most log boilers are most efficient running at full power and benefit from having a buffer tank feed to store any excess heat, rather than connecting direct to radiators.

So, it appears my renewable heating project needs to go a stage further, before I will be satisfied with it.