Adding Solar and additional thermal store

[gnattycole]

Hi,

I'm fairly new to the renewables field but having recently moved into an older property on an exposed coastal site, I've started looking into various possibilities for improving both efficiency and energy storage. The house is a detached upside down style with a stepped footing on a sloping site and being almost South facing with a large glazing area, solar gain is considerable. We have installed a 4KW grid tied solar PV system just over 2 years ago and I fitted an Immersun unit at the same time. Last year I replaced our traditional gravity DHW cylinder with a Telford 280Litre Thermal Store. This enabled me to capture much more of the spare energy generated by the PV array with the TS immersion heater mounted low in the cylinder compared with the old top mounted cylinder but even this is normally satisfied by lunchtime during the sunny summer days.
The Telford has 2 coils, one Solar coil and one for auxiliary heat source, both currently unused but I'm planning to install solar thermal in the near future. The main boiler in the house feeds directly to the feeds on the thermal store and the radiator system is coupled directly to the boiler at present from dedicated tappings and is completely separate with own flow, return, pump and 2 port valve. (Oil fired boiler 120,000BTU). The Telford has 2 additional tappings which are intended to feed heating system and my intention is to use these for supplementary under-floor heating as I install it during renovation work in the near future. So far, since moving into the property, I have halved my first year's annual oil consumption so I guess it is more efficient than the original oil only setup but I would like to extend the Solar capabilities of the house to allow greater energy generation and capture.
Not sure if it's feasible, but I have in my head a plan to install a larger thermal store or buffer tank, something like 5000litre with both immersion and solar thermal coil. The plan was then to use the 2nd output of the immersun, together with a solar thermal array to dump any additional energy into this larger store after the Telford TS was fully charged. I was then considering a piped circuit from the large store through the unused secondary coil in the Telford to transfer heat when the large store had a higher temperature than the Telford, similar to the way a differential controller monitors the delta between the solar panels and store temperature. This will hopefully allow the underfloor heating, together with DHW to draw from the solar energy and the oil boiler can supplement if necessary with the conventional radiators if it gets really cold during the winter months. I'm hoping that the larger store will heat up through the summer months but once it has cooled as the winter drags on, it will become largely dormant and will have little or no effect on the Telford. It's difficult for me to calculate accurately the potential generation capability of the house but as a ball park I have enough South facing roof area for an additional 4KW PV or a mixture of PV and vacuum tube panels. From what I've seen with several friends Solar thermal installations, they tend to over generate during the summer and end up having to run off gallons of hot water to cool the system down and prevent boiling. I would prefer to capture this energy and store it if possible. I am planning to have a garage built next year so space for the secondary large store isn't an issue.
Just wondering if anyone has had any success with coupling two TS in this way or am I urinating into the prevailing? Any info would be gratefully accepted.

 

[Steve]

Rather than running off gallons of water couldn't you discharge into the radiator circuit once the temp reaches, say, 85 degrees? Certainly seems a bad design to me. And not ideal with a water meter.

I'm all for saving energy, but what is the point in capturing more heat than you'll ever need?

I think with the immersun you've got the best setup. It heats your hot water satisfactorily. On your thermal store, is it linked to the radiator circuit? Do this, and then in the winter, you can use the heat from the immersun for your rads before the boiler kicks in. It wont last long, especially in winter, but it'll reduce your space heating bill somewhat.

Water PV will complicate matters in terms of the control, and in effect is using the same energy source.

 

[gnattycole]

Hi Steve, thanks for the reply. My thoughts on the above were not aimed at capturing more energy than I'll ever need, because in the winter, the Immersun rarely satisfies the Telford TS on all but the sunniest of days And tht's just for DHW). My theory of slowly storing the thermal energy in the large TS through the Summer was to enable its release during the winter period. Neither TS will be connected to the radiator system in the house because I don't generally feel that the high temperatures required for efficient radiation from panels suits the setup I'm planning, however, I intend to install underfloor heating in the Lounge, Dining room and Kitchen as these are suspended floors so should enable retro fitting fairly straightforward. I don't really have sufficient headroom in any of the concrete floored rooms to enable the additional depth of pipes, insulation and screed to make it viable, apart from maybe the hallway which is a vaulted ceiling. I have provisioned tappings on the Telford TS ready for connection to the UFH manifolds so hence the thermal energy in both stores will be useful down to a lower temperature. The issue is how to connect the two Thermal stores and control the transfer of heat between the large and the Telford. The house is on a sea facing exposed site so our energy usage for space heating isn't insignificant in the Winter. Anything I can generate from the Sun means another few litres less oil I need to burn. I'll still keep the radiators and oil boiler for back up / top up heating of the rest of the house but figured the UFH would reduce the space heating required in the most occupied rooms throughout the Winter and to be honest, I don't like bedrooms to be too hot anyway. As the system exists presently, the Telford is only providing Domestic Hot Water as I've not fitted the UFH and the radiator circuits are fed from their own pair of tappings on the oil fired boiler, completely independent of the TS.
As I see it in my probably oversimplistic mind, you either need to oversize a Solar system to produce sufficient energy in the Winter and then require some means of dumping the inevitable over production in the Summer, or you size it for Summer balance and then require additional energy support for the Winter months when the requirements are at their highest and the generation capacity is at it's lowest. I'm trying to pitch it somewhere in the middle where I can capture a modicum of excess in the Summer to offset the undercapacity in the Winter. It's as much a hobby project as a mission critical but it will hopefully offer some benefit if I can get it reasonably correct. Feel free to pass on your thoughts though.

 

[Steve]

Even with the world's best insulated thermal store your plan will not work. I've no idea how efficient thermal stores are at retaining energy but the idea of warming up a body of water one month then using it 3 months later sounds completely pie in the sky. Even a thermos flask couldn't achieve this and that has near perfect insulation.