Hi Lisa.
I have abandoned the Hait idea as it is such a massive undertaking. There is a similar idea that has been done, successfully, in Okotoks, Alberta, Canada called Drake Landing Solar Community. I'll let you read about it at the link below.
Having viewed your weather info, [see Ireland weather chart below] you easily could supply virtually all heat needed with passive solar using a R2000 type Super Energy Efficient Build.
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Welcome to Drake Landing Solar Community.
The Drake Landing Solar Community (DLSC) is a master planned neighbourhood in the Town of Okotoks, Alberta, Canada that has successfully integrated Canadian energy efficient technologies with a renewable, unlimited energy source - the sun.
The first of its kind in North America, DLSC is heated by a district system designed to store abundant solar energy underground during the summer months and distribute the energy to each home for space heating needs during winter months.
http://www.dlsc.ca/
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But as you and I are not going to attract a huge community around our own projects, we need a smaller scale idea and that too has come out of this larger project.
You mention lots of water where you are. Do you get sustained freezing temps in winter, does the water, ponds, lakes and the like freeze solid? What are your January temps, night time lows, day time highs?
An engineering department at an Ontario university has scaled this down for single family dwellings. The Drake Landing solar collection and storage is huge and this idea does not scale down well, ie. efficiently for single family homes.
I'll locate the above and post later.
So instead of using the Earth for a heat storage medium, a buried, super insulated water tank, concrete I think, stores the heat collected thru the hot months for delivery in the cold months. Now remember, Canada is butt cold, whereas your heating demands would be much lower. The houses are insulated to a very high standard and uncontrolled air infiltration is effectively zero, with air to air heat exchangers used to ensure a warmed/recycled air exchange.
My initial thinking is that your climate is VERY conducive to creating a super energy efficient dwelling that would have a very low energy load [compared to Canada, northern usa, ... ] and so would require minimal heating.
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What is the temperature of Ireland by month?
Temperature:
January 5°C/ 41 °F 5°C/ 41 °F
April 8°C/ 46 °F 11°C/52 °F
July 15°C/ 59°F 14°C/57 °F
October 10°C/ 50°F 7°C/44.6 °F
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Creating large energy resources is expensive, it also has equipment requiring maintenance, replacement costs while creating a super energy efficient house can last a lifetime.
If your area of Ireland is anything like the above chart I copied off the internet, you can supply a large fraction of needed heat in an R2000 super insulated, recycled air with an air to air heat exchanger style home.
Check out the link below. Then we can talk more when you supply info as to your heating requirements as requested above.
Details of the R-2000 Standard
https://natural-resources.canada.ca/energy-efficiency/homes/professional-opportunities/become-energy-efficient-builder/details-the-r-2000-standard/20588