(in Russian, “hot box”), an arrangement using solar radiation directly for heating water, drying fruit, vegetables, and materials, distilling mineralized waters, and other purposes.

A solar heater is usually made in the shape of a flat rectangular box, often fitted with glass that is either airtight (for heating and distilling water) or has vents (solar dryer) and is turned with the heat-receiving surface toward the sun’s rays. The effectiveness of a water heater of this type depends upon the size of the heat-receiving surface and the temperature of the heater; its output is as much as 50–60 liters per day for each square meter when heated to between 10°-15° C and 50°-55° C. Its efficiency is 0.2–0.5. In southern regions of the USSR with a large number of sunny days annually, a solar power plant of this type saves fuel and considerably cuts the drying time for vegetables and fruits; in addition, both the quality and content of ascorbic acid (vitamin C) are greater than with natural drying. In the central and northern regions of the USSR the same principle is used for hotbeds and greenhouses, devoted to raising vegetables. With constant daytime orientation toward the sun, the output of the device increases by 40–45 percent.