Natural convection loops providing overheat protection in flat-plate collector and integral-collector storage solar water heating systems.

摘要

Solar water heaters made from low-cost commodity polymers show promise for significantly reducing collector costs. However, maximum operating temperatures for these materials are typically ∼80°C, obviating their use in glazed collectors unless some form of overheat protection (OHP) is used. Due to potential for low cost and reliability, increasing the collector loss coefficient through external natural convection loops (i.e., venting) is investigated to obtain actual OHP performance data for a range of vent configurations, develop a generalized and validated modeling approach that can be used to predict OHP performance, and recommend design optimizations.; A polymer integral-collector-storage (ICS) solar water heater with an innovative vent OHP system with complex flow channels under, over, and through the storage reservoir has been tested, along with vent modifications that progressively remove flow restriction until the vent consists of two inclined parallel plates vented along the entire top/bottom edges. OHP performance was measured with and without the glazing insulated (natural convection literature suggests the latter case is representative of venting the collector bottom).; Performance for the original vent configuration was poor; with the glazing uninsulated, the vent increased overall ICS heat loss by only 8%. Modifying the vent to remove flow constraint increased overall heat rejection to 25%; further simplifications increased heat loss to over 50%. Removing the glazing (i.e., upper limit on vent performance) increases losses to almost 90%. For the insulated glazing, activating the vent increases total collector heat loss by ∼60%, 100% and 200%, respectively.; Given the dramatic difference that design variables have on OHP performance, it is critical that OHP heat rejection be predicted in the design phase. A simplified, generalizable and validated vent OHP model is presented that balances buoyancy driving forces against frictional restricting forces, using existing friction correlations from the literature. The model predicts OHP heat rejection well for a wide range of collector configurations. A global correlation relating overall OHP heat rejection to key design variables of vent length, depth, tilt and additional flow constraint is presented. Finally, recommendations are made on how to optimize OHP heat rejection.