Thermal stratification in the accumulator tanks
Journal Name:
- Dicle Üniversitesi Mühendislik Fakültesi Mühendislik Dergisi
Keywords (Original Language):
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Abstract (2. Language):
The applications of thermal stratification are used especially in solar energy systems to store heat in high quality, the accumulator using as a storage tank for hot water which is required in all kinds of energy storage. The amount of emitted thermal energy from the sun changes with time so heat storage must be applied which enables us to benefit from it for a long period. The accumulators, heat pumps and the water heating system must have the same specifications. If the supply and the exhaust of energy cannot be kept in balance, energy storage will become important for sustainable utilization. This mismatch phenomena generally occurs in the systems with unstable resource or requirement, such as solar energy system, hydraulic power generation, food preservation and so on .
The design of heat accumulators needs heat stratifications. Another reason for wide utilization of water tank is based on the critical effect on balance of energy supply and demand, especially in solar energy systems such as solar domestic hot water (SDHW), thermal energy storage, district solar heating systems, and other unsteady energy used occasionally. Therefore, accumulator plays two main important roles as energy reservoir and redistribution.
As far as solar energy system is concerned, from the lower temperature section of the water storage tank (accumulator), the cold water circulating through the collectors is heated by solar radiation, where it becomes the hot water and returns to the storage tank. If the hot water is allowed to mix with the cold water in the tank, the supplied temperature to the load is lowered and the useful quality of energy is degraded.
The temperature difference between hot and cold water produce density changes which leads to appearance of lift force. Here thermal stratification must be used to reduce the degree of mixing in side , this type of energy storage system with thermal stratification is closely related to the efficiency of system. Different designs are available and currently used for tanks that apply thermal stratifications. There are different designs currently used in practical applications.
To obtain optimum system performance it is necessary to promote thermal stratification in the storage tank (accumulator) and to minimize pumping power for the collector loop. The most effective way of promoting stratification is to avoid designs based on water jets entering the tank. A range of stratification promoters or diffuser manifolds mounted inside the storage tank
Thermal stratification requires a controlled charging and discharging procedure, and appropriate methods or devices to avoid mixing; design of storage system is complex.
Thermal stratification in the tank has a great impact on the thermal performance of SDHW systems. A high degree of thermal stratification increases the thermal performance of solar hot-water systems because the return temperature to the solar collector is lowered. A lower return temperature to the solar collector will increase the efficiency of the solar collector.
Many experimental and numerical investigations have been presented for analyzing the performance of vertical thermal storage tanks whilst horizontal cylindrical tanks have received limited attention. The geometry is more susceptible to degradation of thermal stratification due to its small vertical dimension relative to its horizontal performance analysis.
The thermal stratification is well preserved as the cold inflow mixes with the bottom layers. The temperature of the top portion of the tank is gradually reduced due to heat conduction down the wall, mixing induced by hot water extraction and heat loss to the ambient, although the latter is minimised by externally insulating the tank. A negligible increase in temperature of the bottom cold layers is due to the thermal diffusion (at a small scale), and the conduction in the walls see Fig(1).
In this study the structure and design of thermally stratified water tank (accumulator) are studied in addition to theoretical and practical studies which are used to develop heat stratifications in heat accumulators are carried out in this review
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Abstract (Original Language):
Yeterince ısı depolamak ve bu ısıyı yüksek kalitede kullanmak amacıyla yapılan ısıl tabakalaşma uygulamaları, özellikle güneş enerjili sistemlerde olmak üzere, sıcak su depolama tankını içeren her türlü enerji depolama alanında gereklidir. Güneşten gelen ışınımın özellikleri, zamana bağlı değişim göstermektedir. Dolayısıyla ısıl enerji depolama, ısıtma yükünün uzun süreli çalışmalarda kullanılmasına olanak sağlar. Aynı özelliğin, ısı depolama akümülatörlerinde de olması gerekir. Yeni uygulamalarda, özellikle ısı pompalı su ısıtma sistemi akümülatörleri ve sıcak su boylerlerinde bu tür tasarımlara gereksinim duyulur.
Isıl tabakalaşma, sıcak ve soğuk su arasındaki sıcaklık (dolayısıyla yoğunluk) farklarından doğan kuvvetlerin etkisiyle oluşarak, depo içerisindeki suyun karışma etkisinin en aza indirgenmesini sağlar. Bu tip bir enerji depolama sisteminin verimi, ısıl tabakalaşma konusuylayakından ilişkilidir.
Isıl tabakalaşma konusunun ele alındığı, çok çeşitli tank tasarımlarının incelendiği çalışmalar mevcuttur. Halenpratik uygulamalarda kullanılan birbirinden farklı tasarımlar söz konusudur. Bu çalışmada, çeşitli ısıl tabakalı sıcak su tanklarının (ısı akümülatörlerinin) tasarım ve yapısal özellikleri, ısı akümülatörlerinde sıcaklık tabakalaşmasının incelenmesi ve daha önce gerçekleştirilmiş bazı teorik ve deneysel çalışmalar derlenmiş ve irdelenmiştir.
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