State of the art of seasonal storage sy stems of solar energy for house heating
Nowadays, most buildings meet thermal loads using equipments and systems that generate or remove heat when building loads exist. Thermal energy storage (TES) systems enable buildings to meet heating and cooling loads using energy produced at another time.
TES can be designed for storing and providing energy on three basic timescales: diurnal, weekly, and seasonal. The seasonal thermal energy storage (STES) systems enable a building to use heat collected during the summer to heat the building during the winter, or to use cold collected during the winter to cool the building in the summer. Relatively to diurnal storage systems, STES require a much larger total size, while the time scale of charge and discharge is enlarged.
STES systems consist of several components, including the heat (or cold) source, the heat exchange system, the thermal distribution system, the thermal storage medium and the thermal loads. A well known disadvantage of sensible or latent TES is the fact that they have to be at a higher (or lower) temperature than the ambience. Due to this temperature difference they are able to operate as heat (or cold) storage. A thermal insulation is thus necessary to avoid losses over the storage period.
A key issue in the design of a thermal energy storage system is its thermal capacity – the amount of energy that it can store and provide. However, selection of the appropriate system depends on many cost-benefit considerations [Gil A. et al. 2010].
The cost of a TES system mainly depends on the following items:
• The storage material itself
• The heat exchangers for charging and discharging t he system
• The cost for the space and/or enclosure for the TES
From the technical point of view, the crucial requirements are:
• High energy density (per-unit mass or per-unit vo lume) in the storage material
• Good heat transfer between the heat transfer flui d (HTF) and the storage medium
• Mechanical and chemical stability of the storage material
• Compatibility between the HTF, heat exchanger and/ or storage medium
• Complete reversibility for a large number of char ging/discharging cycles
• Low thermal losses
• Ease of control
The most important design criteria are:
• Nominal temperatures
• Maximum load
• Operational strategy
All these facts have to be considered when deciding on the type and the design of thermal storage. Lots of methods and systems have been developed for STES systems. The purpose of this chapter is to identify and selectively review previous work done on the evaluation and use of TES. Appropriate storage concepts and technical options are discussed, followed by a review of previous work. TES systems can be classified by the storage mechanism as sensible storage, latent storage, sorption storage and chemical reaction storage.
Sensible thermal storage
For the sensible thermal storage, the thermal energy is stored by the change of the temperature of the storage medium. Thus, the storage capacity depends on the temperature change, the specific heat and the quantity of storage medium.
The storage capacity of a sensible thermal storage system with a solid or liquid storage medium is given by:
Q MCpT VCpT Eq.1-1
where M is the mass; V is the volume; Cp is the specific heat; is the density and T TmaxTmin is the temperature difference between the maximum and minimum temperatures of the medium. This temperature range depends on the application and is limited by the temperature of the heat source and of the delivery temperature from the storage. This expression can be used to calculate the mass and volume of storage material required to store a given quantity of energy.
Sensible heat storage may be classified on the basis of the heat storage media as liquid media storage (like water, oil based fluids, molten salts etc.) and solid media storage (like rocks, metals and others).
Sensible storage by liquid form
At moderate temperature, water is one of the best storage media. It has higher specific heat than other materials, and it is cheap and widely available. However, due to its high vapor pressure, it requires costly insulation and pressure withstanding containment for high temperature applications. Water can be used over the wide range of temperature of 25-90°C. For a 60°C temperature change, water can store 250 kJ/kg or 2.5 x 105 kJ/m3 [Amaya V. Novo et al. 2010]. Because of its simplicity, a large amount of published data is available on the design criteria for water storage media [Ucar A. 2005, Bauer D. 2010, Kübler R. et al 1997, Tanaka H. et al 2000]. The schematic diagram of a solar heating system with seasonal storage system by water can be found in Figure 1-1. The thermal energy which comes from the solar collector is stored in the water store by the increase of its temperature during the summer phase and is used for house heating during the winter phase. A heat pump has to be used to promote the thermal energy quality for higher temperature using.
Water storage tanks are made from a wide variety of materials, like steel, aluminum, reinforced concrete and fiber glass. The tanks are insulated with glass wool, mineral wool or polyurethane. The size of the tanks used varies from a few hundred liters to a few thousand cubic meters.
Storage by solar pond
A solar pond is a pool of saltwater which collects and stores solar thermal energy. The saltwater naturally forms a vertical salinity gradient also known as a « halocline », in which low-salinity water floats on top of high-salinity water. The layers of salt solutions increase in concentration (and therefore density) with depth. Below a certain depth, the solution has a uniformly high salt concentration as shown in Figure 1-2 [V. Velmurugan et al. 2008, Solar Gradient Solar Ponds, 2009].
Table of contents :
CHAPTER 1 – STATE OF THE ART OF SEASONAL STORAGE SYSTEMS OF SOLAR ENERGY FOR HOUSE HEATING
1.1 SENSIBLE THERMAL STORAGE
1.1.1 Sensible storage by liquid form
1.1.2 Sensible storage by solid form
1.2 LATENT THERMAL STORAGE
1.3 STORAGE BY CHEMICAL REACTION
1.4 STORAGE BY SORPTION
1.5 CONCLUSION OF THE STATE OF ART OF SOLAR HEAT STORAGE
REFERENCES OF CHAPTER 1
CHAPTER 2 – SEASONAL STORAGE OF SOLAR ENERGY FOR HOUSE HEATING BY ABSORPTION TECHNOLOGY
2.1 PROCESS DESCRIPTION AND STATIC MODEL
2.1.1 Process description
2.1.2 Static model
2.2 PRINCIPLES FOR CHOOSING THE ABSORPTION COUPLES
2.3 STATIC SIMULATION OF THE PERFORMANCES OF SEVEN ABSORPTION COUPLES
2.4 MAIN EVALUATION POINTS
2.5 CONCLUSION ON THE EVOLUTION OF THE ABSORPTION COUPLES PERFORMANCE
REFERENCES OF CHAPTER 2
CHAPTER 3 – EXPERIMENTATION OF THE SEASONAL STORAGE SYSTEM WITH THE CACL2/H2O COUPLE
3.1 CONCEPTION OF THE PROTOTYPE
3.2 DESIGN OF THE COMPONENTS OF THE PROTOTYPE
3.2.1 The absorption couple of the experimentation
3.2.2 The materials used for the prototype
3.2.3 Design and description of the devices
3.3 PLANNING OF THE EXPERIMENTATIONS
3.4 RESULTS OF THE EXPERIMENTATION
3.4.1 The desorption phases
3.4.2 The absorption phase
3.5 CONCLUSION OF THE EXPERIMENTATION
REFERENCES OF CHAPTER 3
CHAPTER 4 – DYNAMIC SIMULATION OF THE PROTOTYPE WITH THE CACL2/H2O COUPLE
4.1 DYNAMIC SIMULATION MODEL
4.1.1 Model for the desorption phase
4.1.2 Model for absorption phase
4.2 SIMULATION RESULTS AND COMPARISON WITH THE EXPERIMENTATION
4.2.1 Desorption cases
4.2.2 Absorption cases
4.3 CONCLUSION OF THE DYNAMIC SIMULATION
REFERENCES OF CHAPTER 4
CHAPTER 5 – ANNUAL DYNAMIC SIMULATION OF A BUILDING WITH A SEASONAL
STORAGE SYSTEM BY ABSORPTION
5.1 ANNUAL DYNAMIC SIMULATION MODEL
5.2 DIMENSION AND OPERATING CONDITIONS OF THE SYSTEM
5.3 SIMULATION RESULTS
5.4 CONCLUSION OF THE ANNUAL SIMULATION
REFERENCES OF CHAPTER 5