User:Bicycling Benjamin/Solar Air Heat

Solar Air Heat

Solar air heat is a means in which the energy from the sun, solar insolation, is captured by an absorbing medium and used to increase the temperature of a fluid, air being the fluid in this case. In many ways this occurs in the natural environment, but in terms of capturing this energy and using it productively in the human working/living environment we talk of this as a renewable energy source; solar air heat.

Solar air heat is a renewable heating technology used to heat or condition air for building heat or process heat applications. Solar air heat collectors can be commonly divided into two categories: recirculating and transpired.

Recirculating

Functioning in a similar manner as a conventional forced air furnace, systems provide heat by re-circulating conditioned building air through solar collectors. Through the use of an absorber material to absorb the sun’s thermal energy, and ducting air to come in contact with it, a simple and effective collector can be made for a variety of air conditioning and process applications. A simple solar air collector consists of an absorber material to capture radiation from the sun and transfers this thermal energy to air via conduction heat transfer. This heated air is then ducted to the building space or to the process area where the heated air is used for space heating or process needs. Due to varying air-ducting methods, collectors are commonly classified as one of three types: a) through-pass collectors, b) front-pass, c) back pass, and d) combination front and back pass collectors.

In the through-pass configuration, air ducted onto one side of the absorber passes through a perforated or fibrous type material and is heated from the conductive properties of the material and the convective properties of the moving air. Through-pass absorbers have the most surface area which enables relatively high conductive heat transfer rates, but significant pressure drop can require greater fan power, and deterioration of certain absorber material after many years of solar radiation exposure can additionally create problems with air quality and performance.

In back-pass, front-pass, and combination type configurations the air is directed on either the back, the front, or on both sides of the absorber to be heated from the return to the supply ducting headers. Although passing the air on both sides of the absorber will provide a greater surface area for conductive heat transfer, issues with dust (fouling) can arise from passing air on the front side of the absorber which reduces absorber efficiency by limiting the amount of sunlight received. In cold climates, air passing next to the glazing will additionally cause greater heat loss, resulting in lower overall performance of the collector.

A variety of applications can utilize solar air heat technologies to reduce the use of conventional heat sources, such as fossil fuels, to create a sustainable means to produce thermal energy. Applications such as space heating, pre-heating ventilation makeup air or process heat can be addressed by solar air heat devices. Further strides are being made in the field of ‘solar co-generation’ where solar thermal technologies are being paired with photovoltaics (PV) which increases the efficiency of a typical PV system by generating additional useful energy in the form of both electricity and heat.

Space heating for residential and commercial applications can be done through the use of solar air heating panels. This configuration operates by drawing air from the building envelope or from the outdoor environment and passes it through the collector where the air warms from conduction of the absorber and is then supplied to the living or working space by either passive means or with the assistance of a fan.

Solar air heat can also be used in process applications such as drying crops (i.e. tea, corn, coffee) and other drying applications. Air heated through a solar collector and then passed over a medium to be dried can provide an efficient means by which to reduce the moisture content of the material.

Transpired

Ventilation makeup air is required in many commercial buildings where occupancy capacity exceeds a certain amount and/or air quality becomes an issue as required by international and local mechanical codes. With this ventilation makeup air requirement a greater amount of heat is needed to heat the makeup air if drawn directly from the outside. However, energy recovery devices and solar air-collector technology can reduce or eliminate additional heating. By drawing air through a properly designed transpired air collector or an air heater (such as an energy and heat recovery ventilators ERV/HRV), the air can sufficiently be heated to reduce the heating load during daytime operation.

Not only is the transpired air collector advantageous for pre-heating makeup air, but it is also an efficient means to reduce energy costs, because any heat that escapes from the building envelope (inside of the transpired air absorber) is effectively used to heat the air that is being drawn back into the system. Not only can this be beneficial during daytime operation, but at night in many cases the recirculation can be beneficial when coupled with programmable thermostats that lower the temperature set point to reduce energy consumption when the heat load is not needed.