Construction and action of wind power system for heat production, storage and supply: The heat accumulator

Thermal accumulator (Thermal storage) is an insulated container with the working fluid, which is characterized by high temperature of boiling, high heat capacity and/or large enthalpy of phase transition. In a simplified case, the water can be used as a heat carrier. At more precise approach as the coolant the solutions of salts are employed, which have higher boiling point and larger specific heat capacity. Specific heat capacity is the ratio of heat that is absorbed by mass unity of the body (substance) at endlessly small change in temperature, to the temperature change value [J/(kg×K)]. The amount of heat absorbed by a body depends on the heating method and its temperature. Heat capacity may be measured at constant volume and at constant pressure and temperature. At constant pressure a part of heat is used to perform a work for extension of the body. Other part of heat is employed to increase its internal energy, whereas at constant volume all the heat is spent only onto the increasing of internal energy.

In most cases, the thermal accumulator is insulated container with a double-layer construction, made in the form of a Dewar flask (vessel) designed to provide good thermal insulation. It can also consist of several separated reservoirs. Each of these tanks is designed for the storage of different temperature coolant. A well-maintained heat exchange between the coolants in different reservoirs is executed with the aid of pipelines. Heating of coolant in reservoirs is done cyclically, with the growth of solar activity. Heat stored in tanks in the future is used for heating and hot water supply of dwellings or other purposes. The thermal accumulator can be performed in the overland (ground) and underground location variants. The choice of one or another design is conditioned by the specific tasks that are solved at creation of a thermal system.

Water accumulators of heat. Density of water 2О) is 1000 kg/m3. Specific heat capacity is 4,2 kJхkg-1хК-1. Specific enthalpy ice has a heat of fusion 335 MJ/m3. Quantity of heat stored in one cubic meter of water at heating from 200C to 700C is 4,2 kJ/(kgхК)х1000х500C = 210 МJ. The problem is accumulation and storage of large volumes of water. At oscillation of temperature in thermal accumulator the bacteria or flora (fungi) can be developed and that leads to rotting.

Thermal accumulators on the enthalpy of phase transition and enthalpy of formation. It is more rationally to accumulate heat using the physical and chemical features of phase transition of the substances from one aggregative state to other. The enthalpy of fusion is the change in enthalpy resulting from heating one mole of a substance to change its state from a solid to a liquid. The temperature at which this occurs is the melting point. The standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy that accompanies the formation of 1 mole of the compound from its elements, with all substances in their standard states. The bond enthalpy is the energy required to break a chemical bond. It is usually expressed in kJ mol-1, measured at 298 K. The exact bond enthalpy of a particular chemical bond depends upon the molecular environment, in which the bond exists. Therefore, bond enthalpy values are averaged values.

ΔH = ∑ ΔH(bonds broken) - ∑ ΔH(bonds formed)

 

Waterice. Heat of fusion of ice is 335 МJ/m3. Heat quantity necessary to melt ice is almost equivalent to heat quantity, necessary to heat water from 00С to 800С. At heating of 1 m3 of water from 00С to 800С about 336 МJ of thermal energy are accumulated or 4,2 kJ/(kgхK)х1000х800C = 336 МJхm-3. At crystallization of 1 m3 of water the same quantity of heat is released as is absorbed at melting of ice   - 336 МJ. For comparison, at heating of 1 m3 of water from 200C to 700C near 210 МJ = 4,2 kJ/(kgхК)х1000х 500C are accumulated.

Paraffin wax C25H52. The solid forms of paraffin, called paraffin wax, are from the heaviest molecules from C20H42 to C40H82. The temperature of fusion is 37°C. Specific heat capacity is 2,14…2,9 kJхkg-1хК-1. Cost is 1,0US$/kg.

Sodium sulfate Na2SO4 (glauber salt) and Na2SO4 x10H2O. Every molecule of sodium sulfate Na2SO4 bonds 10 molecules of water. At increasing of temperature the sodium sulfate is dissolved in its water with the absorption of heat.  At the temperature of +320С is fused. At lower temperature it is crystallized and emits heat. Heat of crystallization is 1387 kJ/mol. Sodium sulfate Na2SO4 is saved in the waterless state. Maximal dissolution 59 g per 100 ml of water is achieved at temperature of 32,40С. Cost is near 0,25US$/kg. 10 m3 of solution provide to accumulate 7,2 GJ of heat. Reservoir volume (quantity of water) providing accumulation of 7,2 GJ of energy a day is 7,2 GJ/0,210 GJхm-3 = 34,3 m3.

Sodium and potassium nitrite-nitrate mixtures. Many organizations work on the development of new low cost materials with improved thermophysical properties for thermal energy storage at high temperature (more than 2000C) for solar power generation stations. Efficient storage systems for such applications usually demand transfer of energy during the charging/discharging process at constant temperature. This is the reason why the attention of many scientists has been focused on phase change materials (salt as storage media. The system KNO3 + NaNO3 (NaNO3 - 50 %) has been successfully used for high temperature energy storage purposes, concerning electricity generation by solar concentration technologies. Mixture KNO3 + NaNO3 has other desirable characteristics such as chemical stability, low corrosion and hygroscopicity, and commercial availability at low cost.

For example, NaNO3 (7%) + NaNO2 (40%) + KNO3 (53%) solution is fused at temperature 1420С and provide heating to 500…5400С. Enthalpy of phase transition of NaNO3 is 468 kJ/mol. Specific heat capacity is 117 kJхmol-1хК-1. 

Sodium nitrate NaNO3. Enthalpy of phase transition NaNO3 is 468 kJ/mol. Specific heat capacity is 117 kJхmol-1хК-1.  Molecular mass of NaNO3 is 84,99 unified atomic mass units. The number of moles in 1 kg is 11,8. 

Sodium nitrite NaNO


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