Solar dryer performance during hay drying cycle

Thermal efficiency was evaluated for the 32-bale, solar-heated, forced-air dryer for large round hay bales during 1992 at the Middle Tennessee Experiment Station near Spring HiU, Tennessee. The tests were conducted in accordance with the new ASAE Standard S-423 (1993), Thermal Performance Testing of Solar Air Heaters, except mass airflow rate was measured at the entrance to the collector instead of the exhaust. Day-long efficiencies for the 4-day test period in mid- September ranged from 49 to 52 percent which was lower than the design goal of 60 percent efficiency.

Airflow rate was measured in 2.4-m (8-ft) segments of the total entrance width of 19.5 m (64.5 ft). Each segment was 12.5 percent of the total entrance width. Airflow measurements taken over this reduced air entrance width decreased research costs without a significant reduction in measurement accuracy. Preliminary airflow rate measurements at four locations along the entrance width showed identical flow rates within 1 percent except at the extreme ends of the entrance where flow rates were 6 percent higher than at the central part of the entrance length. Volumetric airflow rate per unit area of collector surface was measured to be 0.067 m³sec/m² (13.27 cfm/ft²) which was lower than the design goal of 0.076 m³sec/m² (15.00 cfm/ft²).

The apparatus for measuring compensated air flow rates included a thin plate orifice and a variable speed fan. The fan accurately added back pressure energy lost by the airstream due to pressure drop through the orifice. Thus, "measured air was delivered to the collector entrance at atmospheric pressure (measured to be within 2.5 pascals (0.01 inches of water) of ambient air pressure). An experiment was designed to evaluate the effectiveness of a moisture extraction exhaust fan positioned in the fan room above control doors that sealed the collector exhaust chamber from the dryer fans. At morning start-up, the exhaust fan removed moisture condensation from the collector air ducts prior to dryer fan powerup. This procedure prevented moisture addition back to partially-dry bales before the relative humidity of the fan room air dropped to the desired upper limit of 60 percent.