Storage systems can stay outside
Hot water storage tanks are mostly installed in homes to provide domestic hot water heating and buffer storage for space heating. However, large storage systems that can store solar thermal heat for months take up considerable space in buildings. Researchers have therefore pursued a new approach: they have designed a hot water storage tank that is placed outside. It is particularly efficient thanks to innovative thermal insulation and special charging and discharging.
In Germany, solar thermal systems with solar coverage levels of about 15 to 30 percent have now become established for providing domestic hot water and space heating support. To achieve the highest possible solar coverage, high-volume and simultaneously inexpensive hot water tanks are needed, particularly if it is intended that the heating demand should be largely covered using solar thermal heat. However, the installation of such large hot water tanks within residential buildings leads to a significant loss of space and incurs higher costs. In collaboration with the Sirch Tankbau-Tankservice Speicherbau company, scientists from the Research and Test Centre (TZS) at Stuttgart University's Institute for Thermodynamics and Thermal Engineering (ITW) have therefore designed a series of hot water storage tanks with volumes ranging from about five to 100 cubic metres. These are designed to enable the heat to be stored without loss for several months so that the heat can be used, for example, for space heating. The storage tanks will be placed outside the buildings. It is intended that the specific costs for the large storage tanks, including for the thermal insulation, will range between 750 and 900 euros per cubic metre of water – for example with a total volume of 60 cubic metres.
Efficient thermal stratification
The main priority is to reduce the internal and external losses from the energy storage system. Internal losses are caused by the loss of effective usable heat through the mixing of water at different temperatures within the storage tank. These are known as exergy losses. In order that sources such as solar panels and heat pumps can work as efficiently as possible, a strong thermal stratification must be maintained when charging the hot water storage system so that these heat generators can be operated at the lowest possible temperature level. For this purpose, stratification charging devices are used.
It's the casing combination that matters
If heat is lost through the tank shell, this is known as an external loss. "To avoid these losses, we are utilising highly efficient insulation materials and techniques in conjunction with transparent thermal insulation. This reduces the effective thermal conductivity by approximately fivefold compared with conventional thermal insulations used for hot water tanks," explains Dr. Harald Drück, head of TZS. For this purpose, the researchers investigated the transparent thermal insulation and vacuum insulation.
Together with the researchers, the Sirch company also constructed the inside of the storage tank so that it was more accessible (pictured left). "This can also be implemented with double-walled storage tank structures with vacuum insulation. This enables the storage tank to be easily adapted to changed system conditions such as a larger collector area or another heat source," adds ITW scientist Markus Gerschitzka, who is largely responsible for the project.
Laboratory-scale and pilot storage tanks
Following initial laboratory tests, the researchers first of all developed several small-scale storage tanks with a volume of 1.5 cubic metres on a laboratory scale. Here they examined how, from a production point of view, the insulation volume can be filled with pourable material, which filling material is best suited and which setting behaviour is shown by the material.
In the next step, they developed a pilot storage tank with a volume of 12 cubic metres. They conducted measurements on the storage tank at ITW using a newly built outdoor test rig for large-scale storage systems. The pilot storage tank has a thermal stratification charging device, transparent thermal insulation made of curved glass and a vacuum insulated panel with special filling material. A manhole located in the floor of the storage tank provides subsequent access. This will enable, for example, the installed stratification charging device to be adapted to changing operating conditions in the storage tank.
The outdoor test rig has a diverse range of measuring sensors. In addition to the temperature of the ambient air, they are also recording and quantifying the possible effects of solar radiation, wind and precipitation. In addition, the temperature in the storage tank is being recorded at 45 different points to determine the thermal stratification in the tank.
The researchers are determining the influence of the transparent thermal insulation by measuring the global and diffuse radiation as well as the resulting inhomogeneous temperature field on the surface of the storage tank's outer casing. Gerschitzka adds: "The tests are intended to show whether the transparent insulation can compensate or even more than compensate for the higher heat losses caused by installing the storage tank outside."
In addition to investigating the thermal stratification charging devices, the tests being conducted on the outdoor test rig are also being used to develop the basis for a standardised approval process for large-scale storage tanks. Further study aspects concern the scalability of passive stratification charging devices. The direct comparison of laboratory measurements with measurements made on the pilot storage tank will enable the researchers to determine the extent to which their findings can be transferred and the scalability of individual sizes.
Further developing storage systems for apartment buildings and solar district heating systems
To enable the outdoor hot water tank to successfully compete in the market, it must be ensured that it is long-lasting, reliable in operation and can be used flexibly and easily installed. To this end, the scientists are researching new container materials and other innovative thermal insulation concepts for hot water tanks that meet these requirements. In addition, the researchers want to develop the StoEx hot water tank presented here and investigate whether such storage tanks are suitable not only for apartment buildings and industrial applications but also for small-scale solar district heating systems.