Using large super-insulated water tanks to inter-seasonally store solar hot water is popularly known as the Sonnenhaus concept in Germany, Austria and Switzerland. The water tanks vary in size from 40,000 litres for a standard house to up to 200,000 litres for apartment buildings and usually cost between € 1.50 - € 2.00/litre.
Sonnenhause owners boast heating bills 50% lower than the occupants of Passive Houses. The Solar panels and Water tank can be a costly addition to the construction budget and they can take up quite a bit of room.
In order to explore this concept a super-insulated 10,000L Hydro Thermal Energy Store (below) was connected to 24m2 of Solar tubes in Cork in 2008, this reduced the clients heating/hot water bill by 65%. 24m2 of drain-back solar thermal tubes heat the water in the tank which reaches over 90 degrees most summers. A 12V PV powered pump fills the drain-back solar tubes with water when the sun shines, so there is no need to use antifreeze.
If the client replaced his low grade PVC windows, the heat-loss from the house would be reduced, allowing for a higher % of heating from solar. When the temperature of the tank drops to 30 degrees it can no longer supply heat to the house due to losses from the heat exchangers. The heat is distributed through the house through an under floor heating array but wall heating pipes will be installed so the house can be heated using lower temperature water, pushing it towards the 75% Solar heated mark.
A 100mm layer of insulation between the floor slab and the Solar Slab allows heat to filter into the living space when the temperature drops in the house. We're presently building a Passive House with a 90m3 sand/gravel Solar Slab within the insulated fabric, the specific heat capacity of the Solar Slab is similar to 30,000 litres of water. The slab will deliver 2kWh for 100 days into the Passive House which is double the heat requirement of the Passive House calculated using the PHPP. Excess mid-summer heat from the Solar Panels will be dumped beneath the house using a heat exchanger, this will raise the temperature beneath the house to 25-35 degrees and reduce the heat loss to the ground from the Solar Slab, this will be used to pre-heat hot water. We've calculated the additional cost of a Solar Slab to be € 30/m2 floor area.
A 100mm layer of insulation between the slab and the battery allows heat to filter into the living space when the temperature drops in the house. We're presently building a Passive House using this method with 90m3 of stone within the insulated fabric, it has a similar specific heat capacity to 45,000 litres of water. The slab is calculated to deliver 2kWh for 100days into the Passive House which is exactly the heat requirement of the Passive House calculated using the PHPP. Excess summer heat from the Solar Panels will be dumped beneath the house using a heat exchanger, this will raise the temperature beneath the house to 25-35 degrees and reduce the heat loss to the ground from the Solar Slab. We've calculated the additional cost of a Solar Slab to be € 40/m2 floor area.
This weekly thermal energy store can store the heat from one day's winter sun in the fabric of a building for up to a week. The temperature release can be slowed down by using 50mm of insulation inside the block. Heating the cold side of a block wall with just 22 degrees solar heated water can deliver a zero heat loss wall.