Seasonal solar thermal energy storage (SSTES) technology for space heating and sanitary hot water systems of residential neighborhood or multilevel buildings.

Prerequisites

Currently, thermal energy generated from thermal power plants (TPP) connected with outdated low-efficiency district heating systems (DHS) in order to deliver generated energy for heating and hot water supply (DHW). DHS have a high level of heat loss due to deterioration of equipment and lack of maintenance. For example, the average specific heat consumption of a multistory residential building is about 270 kWh/m2 per year, while in western countries with a similar climate it is 120-140 kWh/m2 per year.

As a consequence, such elevated consumption of fossil fuels resulted in extensive environmental contamination and significant public health problems in former Soviet Union countries, including, Kazakhstan.

The proposed technology is aimed at using the thermal energy of the sun for heating and hot water systems for residential areas, multi-storey buildings, greenhouses and can be regarded as a centralized heating system.

At the same time, the problems associated with the daily and seasonal variations of the solar energy intensity that make it difficult to use for such intensive systems will be resolved. Thus, the technology will be designed so that it will supply 70-90% of the required heat for heating and DHW systems from the energy of the sun.

Brief technology description

  1. System of flat plate solar collectors
  2. Short term storage based on high energy density materials
  3. Borehole thermal energy storage (BTES) based on single U-type borehole heat exchanger
  4. An additional boiler for the case where the system does not sufficiently cover the heat loads of the heating and DHW systems.

During hot days, heat harnessed using solar collectors is transferred to the STS and then to the subsurface using U-type borehole heat exchangers (BHE) installed into boreholes, and the ground serves as storage medium. The BTES field is circular and insulated on the top. Besides the BHEs are joined together on the surface such that heated fluid from STS charges the ground starting from the centre circulating thought the connected BHEs until the field edge, and flows back to the STS. Hence, the core temperature of the

BTES field is the highest and maintained around 70C. During upcoming cold season, the heat is extracted from the ground using BHEs to the STS where temperature is designed to be around 65C, which is enough for SH and DHW systems of modern buildings. However, if necessary, temperature of the heated

fluid can be further increased by boiler/heater up to required degree. During warm seasons, the energy for SHW system is discharged directly from the STS where temperature is at least 80C. Thus, CSH system can supply around 90% (70%) of the heating requirements for modern buildings (aged buildings) with solar energy.

Functional purpose

The SSTES technology is designed to store solar thermal energy in large amounts in an underground heat accumulator and short-term storage accumulators for the further use of the stored thermal energy in space heating and hot water supply.

The proposed technology solves the problems associated with daily and seasonal variations of solar radiation intensity, which allows the proposed technology to function as a centralized heat supply system all year round, regardless the time and weather conditions.
The technical result consists in increasing the coefficient of average seasonal use of solar energy, which makes it possible to supply more than 70% of thermal energy necessary for SH and DHW supply  due to the solar energy.

Technology application

The proposed technology is a centralized system for solar thermal energy storage and for its further application in space heating and domestic hot water supply systems.

Features and advantages

As a result of the project, the technology of seasonal solar thermal energy storage system (SSTES) for space heating and hot water supply of premises/residential buildings will be provided.
A) One of the advantages of the technology is its environmental friendliness, as the use of solar energy is implied. Thus, the SSTES technology can replace thermal power stations operating on environmentally hazardous fossil fuels.
B) SSTES is easily scalable, since any number of Borehole Heat Exchangers, STS accumulators and solar collectors can be integrated to the main SSTES system if more nearby buildings are being added for centralized heating system on the basis of SSTES technology.
C) The system is additionally equipped with an intelligent control unit (IntCU), which has access to temperature, water-level and pressure sensors; flowmeters; valves and pumps. Depending on the signals received from the sensors, it is possible to remotely control the shut-off valves, the flow of heat carriers in the heat exchangers, turn on/off solar collectors, pumps, or change the direction of flow in the pipes.
D) The majority of STS accumulators on the market use water as a storage medium. Water can store up to 225 MJ of thermal energy per each cubic meter when water temperature changes from 25°C to 80°C, to 55 ° C. For comparison HDTES materials, developed by the authors, can store about 600 MJ of thermal energy per cubic meter with a smaller temperature change from 20°C to 30°C. Therefore, STS accumulators based on HDTES material allow to store more energy in smaller volumes.
E) the working life of SSTES system is 20 years and it can operate more if it is installed correctly.
F) Seasonal thermal energy storage on the basis of a geothermal accumulator is relatively inexpensive compared to other large-scale storage systems, such as Thermal Energy Storage Tank (TEST) or Aquifer Thermal Energy Storage (ATES). In the case of TEST, where the storage medium is water, there is a high probability of water leakage problems. And the performance of the ATES systems depends on (I) the chemicals composition of the groundwater that often cause corrosion of the metal equipment and piping system when circulating through the system and (ii) the rate of groundwater that can wash away the stored thermal energy. Another problem, (III) is the clogging of hot/cold water wells, since the soil or rock usually easily enters the open circuit of the ATES circulation.
And when energy is stored in a geothermal accumulator, (SSTES case) the circulation of the water occurs in a closed loop, that is why, there is no direct interaction of groundwater with the water circulating in the system, which indicates that the above-mentioned problems are not found in the geothermal accumulator.
G) SSTES works at low pressure (1-3 bar), and medium temperature (25oC -90oC), and uses non-toxic materials, which ensures environmental friendliness.
H) The system of solar collectors is connected to the STS accumulators via a heat exchanger. Such a connection makes it easy to replace the solar energy with any other source of low-potential heat, such as the waste heat of industrial plants and power stations.

Existing companies and projects in Kazakhstan with similar strategies or proposals:

  • Use of solar energy in Kazakhstan is insignificant, despite the fact that the annual duration of sunlight is 2200-3000 hours per year, and the estimated capacity is 1300-1800 kW per 1 square meter per year.
  • In 2010, a project for a fully vertically integrated production of silicon-based photoelectric modules was launched in Kazakhstan. KazSilicon produces silicon in Ushtobe city (Almaty region).
  • “Kazakhstan Solar Silicon” LLP in Ust-Kamenogorsk processes raw materials and produces solar panels. The company “Astana Solar” realizes the last stage of redistribution and assembly of PV modules.
  • In the end of 2012 in Kordai which is in Zhambyl region, the first solar electricity station “Otar” with a power capacity of 504 kW and an energy capacity of 7 MW was given for exploitation.
  • On December 20, 2013, during the national teleconference “Building Together a Strong Kazakhstan!”, the Kapshagai Solar Energy Station (Kapshagai, Almaty region) with a capacity of 2 MW was launched. This plant uses technology for tracking the sun. The project implemented by the subsidiary company of JSC “Samruk-Energo”, LLP “Samruk-Green Energy”.

According to the information given above, at the moment, there are no active projects similar to SSTES in Kazakhstan. Therefore, the idea of ​SSTES technology does not have significant competitors on the market. A similar analysis that was carried out in the markets of former Soviet Union countries revealed that such technologies are not developed there either, which means absence of competitors. Geothermal seasonal energy storage facilities using a heat pump are available in the European markets. Its price ranges from 300000-720000 € depending on the heat load.
The price of the SSTES system will depend on the required thermal load of the multi-level buildings/ micro-district. Despite this, its price will be affordable for customers, since SSTES is based on relatively inexpensive accumulators, and designed for long-term operation. The payback period of this technology is 5-6 years.

The main compound of SSTES technology is the geothermal accumulator that is created by the network of borehole heat exchangers. Consequently, one of the major items of technology costs is drilling, for example, the drilling of one well costs about 1.5 million KZT, while we need to drill at least 12 geothermal heat exchangers. To reduce the cost and shorten the payback period of technology, the acquisition of a drilling rig is planned. Therefore, all works related to drilling, namely: pre-installation work to determine the thermal properties and geological features of the terrain, assess the potential of the soil for GTA accumulator installation; and further works on the installation of the GTA  accumulator will be carried out by the technology developers themselves;
As well as design calculations, to find the total number of components included in the system for each specific case; building and equipping of an energy center, which includes a STS accumulator, measuring instruments and control systems.

Since the installation works will be conducted with the participation of the technology developers and the partner company KunTech, the quality of the installation of the technology will be guaranteed. After completion of the installation of the system, all parts of the system will be tested to work properly and a test run will be performed to identify and eliminate possible problems. Only after these stages the technology will be transferred to customers.
Time spent on installing and integrating SSTES with buildings or residential areas depends on the size, number and types of buildings. Thus, during the first 3 years on the market, if there is a demand for large-scale objects, the target number of installed SSTES technology will be 3 units per year, whereas for medium-sized objects – 6 units per year. After mastering the installation skills of the technology, in view of simplification, acceleration and reduction in the cost of the installation, the number of installed systems will be increased to 5-6 units per year for large-scale objects and 8 units for medium-scale objects. Also, training courses will be organized to train new specialists for further development and expansion of the company.

Innovative aspects of technology

A) A distinctive feature of the technology is the use of HDTES materials in STS accumulators, which, as mentioned above, allow to store more energy in small volumes and with less temperature changes. The composition of the phase transition material depends on the climatic features of the area and the required power load of the user block.

B) The geothermal accumulator, which is the main component of the technology, can be charged and discharged with the thermal energy of the sun. Moreover, the heat pump is not used for heat extraction from the soil. Water is used as a heat carrier. For comparison, a geothermal heat pump system (GTHP) can be considered. Here the natural heat of soil is extracted for consumption by means of a heat pump. However, such GTHP systems are inefficient, since the temperature of the earth around the geothermal heat exchanger is gradually decreasing over many years, reducing the efficiency of the heat pump due to the constant heat collection and insufficient time for restoring the soil temperature naturally.

C) Another distinctive feature of the system is that heat exchanger circulates in the closed loop, that’s why there is no need in installation of filtration and disinfection devices to achieve water quality standards for consumers.

High average density of thermal energy storage: in the STS accumulator it is about 330 kJ/kg (the volume varies from 1-3 m3) and in the geothermal accumulator 60-70 kJ/kg (approximate volume is more than > 8000 m3). Scalability, as any number of Borehole Heat Exchangers, STS accumulators and solar collectors can be connected to the main SSTES system if more nearby buildings or industrial facilities are being integrated to the centralized SH system on the basis of SSTES technology. The heat pump is not used. For charging and discharging the GTA with solar thermal energy the typical water will be used as the heat carrier.

This technology is a conceptually new approach to the storage of thermal energy. Since it is proposed to store thermal energy underground in the soil. The volume of the geothermal accumulator is more than > 8000 m3, and easily can be expanded by drilling some new wells. GTA can be charged and discharged many times with the help of typical water, which circulates in a closed loop of the wells network as a heat carrier.

The use of solar energy for heating and DHW supply is also a conceptually new approach to energy supply. This technology can replace Thermal Energy Stations working with hazardous fossil fuels such as coal or gas.