In this website, various terms are used to describe the dryers, the materials to make them, and how the solar dryers work. The following tries to define what those terms mean:
In describing the various types of collectors, the terminology from Fig. 2 in the VITA document Understanding Solar Food Dryers:
Active Solar Dryer. An active solar dryer collects the solar heat in one part and then uses some mechanical means (typically a fan or a pump) to move the heat to the drying food.
Passive Solar Dryer. A passive solar dryer either collects the heat right where the drying food is, or uses convection driven by the collected heat to move the heat to the drying food. In general, the focus of the work at the ECHO East Africa Impact Center has been on passive solar dryers.
Passive solar dryers are divided into two types:
Direct solar dryers where the food is exposed to direct solar radiation.
Indirect solar dryers where the solar energy is collected in one part and transferred through convection to the drying food.
In describing the solar dryers, the following terms are used to describe parts of the dryer:
Glazing. The transparent cover that lets light in and slows the loss of heat.
Collector. The area covered with glazing that captures the suns energy.
Drying chamber. The area where drying food is put in the dryer. In an indirect dryer, it is separate from the collecter, but in a direct dryer it is the same space as the collector.
Drying trays. Flat trays used to hold the drying food in the drying chamber, Usually they have some kind of screen to hold the food.
Ventilation openings. Holes that let cool air in (usually low in the dryer) and heated, moist air out (usually higher in the dryer).
Chimney. An element (usually vertical) designed to capture some of the suns energy to enhance the air flow (also known as the draft) of a solar dryer by heating air with the sun in added space specifically for this purpose.
Insulation. Insulation, a material added to the walls and floor of a dryer to keep heat in, may come up in discussions of solar dryers. Because of the overall warm temperatures in Tanzania and East Africa, and because inexpensive insulation that is easily incorporated into the dryers is not widely available, the current models at ECHO East Africa do not use insulation.
In the references listed in this web page and some of the discussion, terms describing the various ways energy is transferred might be used:
Conduction. In conduction, heat is transferred from on material to another by direct contact. Usually this is solid materials but it can happen with liquids and gases.
Convection. This is when heat is transfered from one location to another by the movement of a heated liquid or gas. It often takes place naturally when the heated or cooled liquid changes density and rises or falls, but it can also be driven by pumps and fans.
Radiation. Electromagnetic radiation can move energy from one object to the other. The suns rays are one example. While some of the energy transfer by radiation is visible to us when the energy level of the radiation matches the sensitivity of our eyes (we call this the visible spectrum or light), there is also a lot of energy transferred in the infrared spectrum where the wavelength is longer than what our eyes can see. Infrared (IR) energy transfer is very important in many solar energy efforts, including solar drying. Also important is ultraviolet radiation where the light wavelength is shorter than our eyes. Over time, ultraviolet or UV radiation can damage many materials, including those used to make solar dryers, as well as biological materials like our skin or the foods we are trying to dry.
Latent heat flux. Latent heat flux (movement) is also important in food drying. When moisture evaporated, going from liquid to vapor, it absorbs a lot of energy with changing the temperature. And when water vapor condenses to become liquid water, it also gives up a lot of moisture.
Related to radiation, and solar energy, there are various terms that come up in technical discussion:
Solar altitude angle. The angle of the sun above the horizon relative to a horizontal surface.
Solar azimuth angle. The angle of the sun in the horizontal direction. It is usually measured relatively a south facing direction in the northern hemisphere and a north facing direction in the southern hemisphere
Angle of incidence. The angle of the sun makes with a surface such as the glazing of a solar panel or solar dryer.
Transmissivity. Transmissivity refers to how much electromagnetic radiation is allowed through a material. It varies with the wavelength and the thickness of the material. Glass with low iron glass lets a lot of visible light through,but blocks some infrared light. This effect is used in solar collectors to capture the sun's radiation (which mostly comes as visible light) by letting it through the glass. But when some of that energy tries to escape as infrared radiation, it is blocked by the glass cover. This effect is called the "greenhouse effect".
Absorptivity. Absorptivity reflects how much light is absorbed when it passed through a material. It is the converse of transmissivity.
Reflectivity. Reflectivity refers to how much electromagnetic radiation is reflected. It is often dependent on the wavelength of the light and the angle of incidence.
Terms related to water content in air:
Relative humidity. The amount of moisture in a volume of air relative to the amount of moisture it could hold at a given temperature. Expressed as a percentage.
Water content of air. The mass of water existing as water vapor in a given volume of air.
Dew point. The temperature at which water vapor will begin to condense on a solid surface. The relative humidity will be 100% at the dew point.
Latent energy. The energy that would be released when the water vapor in air condenses, or which will be absorbed if water vaporizes. Also called enthalpy.
Psychrometric chart. A graph (see this example ) showing the interrelationships of temperature, water content, density, and latent heat.
Terms related to measuring how dry a food is:
Percent moisture. The water content of a food as a fraction of the dry matter content of the food. Expressed as a percent. In some materials, it can be measured with electronic instruments, but to determine it by weighing you need to weigh dry the food before and after drying it for about 24 hours at 105 to 110° C. The percent moisture is the weight loss divided by the final weight (aka the dry weight), and multiplying the fraction by 100.
Percent moisture removed. One can calculate the percent moisture removed from a food by weighing it before and after drying in a solar dryer. The percent moisture removed is then the change in weight divided by the original weight of wet food, and the multiplying by 100. This quantity is not the percent moisture of the food.