Executive Summary : | Solar parabolic trough
collector is most proven technology and commercially available for power generation with high dispatchability. The solar radiation incident on the parabolic concentrator is focused on the receiver which converts the solar radiation into useful heat energy. High boiling working fluid (synthetic oil) is circulated through the receiver to collect concentrated solar energy from the receiver. The heated oil is passed through a series of heat exchangers to generate the steam for producing electricity. The major drawbacks of synthetic oil as heat transfer fluid are (a) limitation on maximum operating temperature, (b) degradation of thermal properties over the period of time, (c) poor heat transfer characteristics and (d) high cost. Also, it requires additional heat exchanger to generate superheated steam. It results in additional investment and lowers the overall plant efficiency. These limitations may be overcome by producing steam in the solar field itself and such an approach is called direct steam generation (DSG). DSG provides opportunity to increase the solar field operating temperature beyond 400oC, improves the receiver and overall plant efficiency and reduces the capital investment by eliminating the additional heat exchanger. Though, DSG approach for solar thermal power generation offers the above mentioned benefits, it also has several challenges. DSG in the receiver involves two phase boiling heat transfer and it has different flow regimes based on the flow rate and steam quality. Two phase stratified flow occurs in the receiver and it results in higher angular thermal gradient across the receiver. It leads to thermal instability of the receiver which results in deflection of receiver from the focal plane. This results in poor intercept factor and rupture of the glass tube. One of the approaches suggested for overcoming the above mentioned problems is to have micro-grooves at the inner surface of the receiver. Micro-grooves help to early transition of stratified to annular flow due to increase in wetting angle of the liquid by capillary action. The micro-grooved tubes also increase the heat transfer by (a) thinning of liquid film, (b) large surface area and (c) increased turbulence. Currently, micro-grooved pipes are used mostly for refrigeration application. The wetting angle and liquid front position depends on thermo-physical properties of the fluid and pipe, operating conditions of the fluid and heat flux boundary conditions. Hence, detail study required to investigate the effect of micro-grooves on wetting angle and liquid front position for DSG in the solar linear concentrators. In this project, detailed numerical and experimental investigation of DSG in a smooth and micro-grooved horizontal absorber will be carried out. The proposed micro-grooved absorber may be used for DSG in the solar parabolic trough collector and linear Fresnel reflector system. |