Cellular concrete & water as a new heat storage medium for underground installation improve all-seasonal effectiveness of solar thermal and other eco heat energies

Background

Buildings today account for 40 percent of energy consumption in developed countries according to the OECD. In view of the energy shortage and desirability of reducing CO₂-emissions, there is a need for means of heating and cooling buildings while conserving energy. The target of the World Council for Sustainable Development (WBSCD) is to be able to design and construct self-sufficient and environmentally sound buildings by 2050. These buildings will use zero net energy from external energy grids and produce carbon dioxide emissions as little as possible, while being economically viable to construct and operate. More information on that topic can be found at http://www.wbcsd.org.

Thus, buildings of tomorrow will require a combination of minimised onsite energy generation incorporating maximised renewable heat energy sources, ultra-efficient building and insula-tion materials and equipment, and waste heat recovery. New alternative heat sources are called "eco energy sources".

Solar energy is one of them. Today solar energy heat collectors are working quite efficiently. However, in order to make practical use of them, it is necessary to have some kind of a heat storage system to maintain a sustainable and steady heating and cooling supply of the building also during nights and periods of cloudy, sunless days.

Other sources like the shallow ground heat certainly have a lower temperature level and need the support of the heat pump, but have a more steady supply quality over the whole year, because it is solar energy pre-stored in the ground.

Waste heat from untreated wastewater from the municipal collection system is another low-temperature heat source. The temperature of the wastewater in thousands of kilometres of municipal sewer lines is 12-16°C on average due to frequently incoming wastewater from urban and industrial processes. It can be up to 20°C or more depending on the geographical location of the particular country, and is nearly constant over the whole year. Furthermore, the wastewater collection system is a huge shallow ground heat horizontal collector and heat storage system with a perfect infrastructure for reaching every resident over a short distance with the mentioned alternative heat.

Japanese estimates indicated that 40% of the waste heat discharged by urban communities is carried away in the sewage. Sewer collection systems therefore contain a large amount of sustainable heat energy with a huge future perspective for every community in the world. And cooling will become a major role in energy efficient houses, so the particular produced waste heat is also a perfect sustainable eco heat source, which can be stored continuously in the underground storage structure during the entire year.

Thus, if a combination of ultra-efficient building materials and eco heat energy sources are used consequently, the onsite heating and cooling request of such a building is relatively low, and therefore a corresponding smaller-built heat storage system is only necessary. And this opens the opportunity, that every new developed real estate ore new-built building or house can have its own separate storage system, if it is easy and cost effective to install. This can generally be provided if it is an underground construction, which can be over-built, and no internal space will be used up. In narrow city right-of-ways or crowded by infrastructure space is limited and/or on private land very expensive. Also, the heat an underground storage structure requires less insulation, because it is covered by the building and embedded in the ground with its own heat potential.

New Construction

Following the above concept, a heat storage system (as in Figures 1 and 2), which is similar to a gravel-water heat accumulator, is in the developing phase. The innovation is, that it contains a cellular concrete material in one solid block with a very special porous structure (after curing) instead of a heap of loose gravel. Therefore, this material needs no heavy-duty and expensive basin, tank or reservoir structure made of concrete or steel etc. No doubt, this reduces the installation costs considerably.

Installation

The installation is very simple. Before the building is erected, an appropriate large and special shaped pit has to be additionally dug up down from the building excavation bottom. The side walls of the pit have to be lined at least with a watertight foil and an appropriate insulation material. The bottom of the pit can be left without sealing, since the liquid bonding agent (cement mortar) penetrates into the peripheral area of the soil and seals the cellular concrete body against the surrounding ground (self-sealing-effect) after curing. The delivered or on-side mixed cellular concrete is poured into the pit and filled up nearly to the top while a heat pipe collector system is integrated and positioned in several horizontal levels.

After the cellular concrete is cured it is filled up with water, and the foundations and the base plate of the building can be build on top, while the collector pipes and the filling and measurement pipes and sensor connections will be vertically inserted. Now the building can be erected. The porous structure and the material property of the cellular concrete sucks up the water until it is full and holds it by the capillary-effect. This has the advantage, that no noticeable volume of water can run out and get lost, if the water-tight foil or the bottom and/or side concrete sealing has any leaks. Lost water can be topped up via the filling pipe installation.

Advantages

• Cellular concrete can be simply produced in large quantities for a reasonable price.
• Cellular concrete is easy to install by minimum use of cement, and fills up the pit completely.
• Cellular concrete structures can take up a high volume of water after curing and hold it captured by capillary capacity, making use of the high heat storage capacity of water.
• Cellular concrete has a high bearing strength, and thus can be over-build without any supporting building elements.
• Cellular concrete has a self-sealing and reinforcing effect towards the surrounding soil.
• The cellular concrete storage system is environmentally friendly, maintenance-free, storage efficient and cost effective, long-lasting, and corrosion resistant.

Example of a renewable (eco) heat supply concept
A One-Family-House (smallest unit) with a cellular-concrete, low-temperature heat storage structure (8) beneath, served with eco heat from sewer heat recovery (1-5), solar heat collectors (6) and in-house waste heat