Concrete & Water — a new heat storage & exchange medium to improve seasonal effectiveness of solar thermal and other alternative heat energies

Background
Buildings today account for 40 percent of energy consumption in developed countries according to the Organisation for Economic Cooperation and Development (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 international topic can be found at www.wbcsd.org.

Thus, buildings of tomorrow will require a combination of minimised onsite energy generation incorpo-rating maximised renewable heat energy sources; ultra-efficient building and insulation materials and equipment; and waste heat recovery.

Solar energy is one of those most potential renewable energy sources. Today, solar energy heat collectors are working quite efficient. However, in order to make practical use of them, it is necessary to run them in combination with a heat storage system to maintain a sustainable and steady heating and cooling supply of the building. This is especially important during nights and periods of cloudy, sunless days, and in the wintertime (seasonal use).

Other sources like the shallow ground heat, the waste heat from untreated wastewater collection systems and from cooling and air-conditioning systems, and the energy from biomass boilers, from micro-cogeneration to heat pumps are further potential eco heat sources, which can and should be buffered in a heat storage system. So heat storage is an essential part of a very broad range of renewable energy and waste heat recovery applications, and is an enabling technology, without it, alternative heating would not be possible. Although heat storage itself is rather invisible, its impact on the amount of renewable energy generated in your house, your city, and your country is huge. With advanced heat storage technologies, it becomes possible to store summer solar heat for the winter, raising the solar fraction to 100%.

Further ultra-efficient building and insulation materials are necessary to reduce the general heat energy request and to make smaller heat storage capacities possible. And this opens the opportunity that every new developed real estate or new-built building or house can have its own separate seasonal storage system, - if it is easy and cost effective to install. And this can generally be provided if it is an underground construction, which can be over-built, and no interior space will be used. In narrow city right-of-ways and private land space is limited and very expensive. Also an underground storage structure requires less insulation, because it is covered by the building and embedded in the ground with its own heat potential.

So every alternative or renewable use of heat should be collected in an appropriate seasonal heat storage system together with a modern house building concept to run it efficiently. And very important is the need to make the distance of energy transportation as short as possible. So appropriate heat storage systems become a key technology function in every alternative heating & cooling system.

News material
Following the above basic concept Orange Depot & Exchange Systems has developed a new heat storage building material, which is a modified kind of concrete/mortar cured in one solid block with a very special porous and capillary structure (see Figure 2). This structure takes up water completely up to over 50 % of its volume and hold it by capillary effect. And this material can be over-built while it needs no additional heavy-duty and expensive basin, tank or reservoir construction. And the material consists of conventional building material components (cement, lime plus additives), so it can be simply manufactured and installed by low cost.

Construction
Two basic variants of construction (A and B, see picture below) are planned so far:

A. Ground-coupled short-term heat storage system
The construction is very simple. Before the building will be erected, an appropriately sized and special shaped pit is excavated. The heat storage system can be installed in two options. One is to deliver the heat storage system as a ready cured concrete block with completely integrated technical equipment by a truck to the site and heaved by a mobile crane into the pit (Variant A-1, see Figure 3).
The second option is transporting the ready-mixed concrete to the site by a truck and filling it directly into the pit (Variant A-2). The concrete has been filled up to the top of the pit while a heat pipe collector system is integrated and positioned in several horizontal levels and/or vertical sections Depending on the planned later specific use of the storage system the side and bottom wall of the pit can be lined before/after the concrete installation with a watertight sheeting (fat black line) and/or an appropriate insulation material (grey coloured with fat black points). After the concrete is cured, respectively the delivered block is set into the pit the concrete can be completely saturated with water. Then an insulation plate made of conventional watertight cellular concrete, or any other technology, and the base plate of the building can be built on top, while the connecting filling and measurement pipes and sensors will be vertically inserted. Now the building can be erected. The porous and capillary structure of the concrete hold the water by itself, so if there is any leak only a trace amount of water is lost, which can be topped up again via the filling equipment.

Variant A-1 is designed to be running as an independent storage system, so it needs an additional watertight sealing and insulation. If as in this case a horizontal collector system is installed, under the bottom of the storage system no insulation is necessary, because in the bottom area the cold heat is charged, which gets warmer at the top of the storage block (up to nearly 90 °C). The insulation around is attached on the site by filling in a conventional cellular concrete or any other appropriate material into the empty space of the pit.
As mentioned, another option is to install a vertically arranged collector system, e.g. by a field of spiral (Slinky-) collectors (see below). The hottest area is in the vertical middle of the storage system, and the temperature fall radially to the side (to the circumference of the concrete block).

Variant A-2 is similar to A-1 but especially designed to be build onsite and to correspond with the surrounding soil and its ground heat and other ground heat exchange and recovering systems. See AFEH plant design Example 2 below. The advantage of this heat storage system is that it needs no additional insulation to the side and downwards. A further advantage is that the ready mixed material can be poured directly into the rough pit while the liquid bonding agent (cement mortar) penetrates into the peripheral area of the soil and forming there a strong concrete wall after curing, which seals and wraps the cellular concrete body with a kind of a shell against the surrounding ground (self-sealing-effect). This concrete shell can be conditioned as a completely watertight sheeting, or a semi watertight concrete wall, whose controlled leaking is used for emitting additional water to the soil to optimise its condition for better ground heat storage and transfer.
But the same function can be reached with the pre-manufactured variant A-1 too, if no insulation is be planned.

B. Vertical spiral-(Slinky-) ground heat collector
Variant B is basically similar as the variant A, but its main designed function is less as a heat storage system but more as an underground borehole heat exchanger, which is additionally wrapped in a water/heat storage material made of concrete. The depth and cross dimension including the concrete coating can vary depending on the individual request and use. The smaller the cross dimension and longer the depth is the more it is a heat exchanger, the bigger the cross dimension and shorter the depth is, the more the system become a heat storage system.

For that special purpose conventional vertical spiral (Slinky-) ground heat collectors are recommended. Their depth into the ground do not reach more than 6-10 m, and the collector pipe is spirally screwed. The bore holes can be simply made with a well-drill under low-cost conditions. These collector pipes have a comparable high heat exchange rate per meter installed length, and therefore be only advised to be designed for both, charging and discharging heat into and out of the ground. So they are the perfect equipment for running a ground heat storage system right under city infrastructures, resp. houses and buildings. Depending how big the ground heat storage system have to be, a field of a number of those collectors need to be installed. See the AFEH plant design Example 1 and 2 below, but can be also used for running a vertical heat pipe exchange system in heat storage systems like variant A.

The main advantage of this new combined heat exchange & storage system is, that the heat exchanger pipe(s) are embedded not in a conventional grouting material like normal cement, soil, or bentonite but in a special concrete material saturated completely with water, which will increase tremendously the heat exchange rate and comfort. And the controlled leaking of the wrapping concrete shell moistens the surrounding Vertical Slinky-Collector soil. Both provides a better condition for heat transfer and long-term (model) (seasonal, annual) ground heat storage capacity. And this type of heat ex- changer can perfectly combined with the heat storage system variant A. See draft AFEH plant design in Figure 5.

The drawing B-1 shows a pre-manufactured spiral (Slinky-)collector ex works completely integrated in a concrete pile with an additional outside mortar sheeting. On the top the pipe connections for the spiral heat pipe (left and right) and the water filling pipe (centre) are arranged.

The drawing B-1.1 shows a pre-manufactured spiral (Slinky-)collector similar to variant A-1 completely integrated in a concrete pile with an additional outside mortar sheeting ready installed in a back-filled bore hole, over-built by a back-fill material (grey) and a watertight perimeter insulation and the basement plate of the building. In this case the depth of the bore hole ends just over the groundwater level. The head of the collector with its pipe connections is additionally bordered by a concrete ring shaft. A duct containing all connecting pipes enter the ring shaft on the side and then be laid under-ground under the perimeter insulation until it is linked to the building through its basement plate.

The drawing B-1.2 shows an on-site installed spiral (Slinky-)collector in a bore hole back-filled with a water saturated concrete. Further details see B-1.1 and variant A-2.

Draft Plant Design of an Alternative-Fuel-Energy-House (AFEH)
This concept includes a modern building erected with energy efficient building materials and a self-sufficient energy supply system by using a low-energy heating and cooling system and renewable and waste heat energy sources as solar heat and power combined with ground heat recovery technology, and if necessary a heat pump. The complete plant is managed by an Energy Management System (EMS).

Example 1 shows an AFEH building with an annual ground heat storage system right under its foundation plate run by a field of 12 vertical spiral (Slinky-)collectors (see variant B), which are additionally integrated in a special water saturated concrete coating. Beside heat exchange and storage this concrete coating has the ability to transpire water continuously to the surrounding soil in a very small dosage, so that the soil gets better and better conditioned for heat storage and transfer but keep its necessary static strength. This design is a simple and effective way to use the complete ground space under buildings for long-term heat storage.

Example 2 shows an AFEH building with a hybrid heat storage system, which combines a short-term heat storage system (variant A) integrated into an annual ground heat storage system run with vertical ground heat exchangers (variant B) right under its foundation plate.
The bore hole ground heat exchangers are arranged around the short-term heat storage system, so no additional insulation to the side is necessary. The soil is moistened by controlled leaching of water out of both, the short-term heat exchanger and the ground heat exchangers. This creates an optimal condition for heat storage even under overbuilt surfaces under most economic aspects. Both examples provides as short as possible energy transportation distances, so heat loss is minimised.

Advantages

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