Intro to Solar Heating

Solar Space Heating Introduction

Solar space heating with air solar collectors is more popular in USA and Canada than heating with solar water collectors since most buildings already have a ventilation system for heating and cooling.

Solar Radiation

For a workable solar energy system, you should understand how the sun’s energy reaches the earth and how this energy varies according to the time of year.

The optimum climatic conditions for solar heating are based on bright sunshine on the coldest days of the year. A solar collector is then able to gather plenty of energy when it’s needed most.

What is surprising is the amount of energy available even on cloudy days, which also tend to be not as cold. Clouds act as a blanket over the earth preventing some of it’s energy from radiating away. Solar radiation reaches solar panels in three ways: as direct, diffuse, and reflected radiation. The three types of radiation are illustrated in fig 1.

Direct radiation consists of parallel rays coming straight from the sun. This type of radiation casts shadows on clear days.

Diffuse radiation is scattered, nonparallel energy rays. This type of radia­tion makes the sky blue on clear days and grey on hazy days.

Reflected radiation is solar energy received by collectors-from adjacent surfaces of the building or ground. It depends a lot on the shape, color, and texture of the surrounding surfaces.

Figure 1. Three types of solar radiation: direct, diffuse & reflected

A nearly constant amount of solar radiation strikes the exterior of the earth’s atmosphere 1,350 W/m2 (429 Btu/h.ft2 ) However, a large amount of this energy is lost in the earth’s atmosphere by absorption and reflec­tion as it travels towards the earth’s surface. The purity of the atmosphere, vapor, dust, and smoke content all have an effect on radiation, as does the angle of the sun. The relative amount of radiation received on earth is diminished when the sun is lower in the sky.

Clouds and particles in the atmosphere not only reflect and absorb solar energy, but they also scatter it in many directions. Thus, part of the solar radiation may be diffused. Diffuse radiation, as opposed to direct radiation, is greater on hazy days than clear ones. Diffuse radiation can account for 50 percent of the total annual radiation for a wall facing south.

Reflected radiation from adjacent surfaces amounts to about 20 percent of the direct and diffuse solar radiation. However, with a bright snow-covered surface in front of a solar collector, the reflected radiation can increase to over 50 percent. Reflected radiation from adjacent surfaces, can be a very important factor in collector sizing and placement.

Figure 2. Sun angle for latitude of 40 N

The sun’s path at the start of summer (June 21) is at its highest position in the sky and the sun is at its lowest position in the sky at the start of winter (December 21).

Collector Angle

Solar designers have traditionally recommended that collectors used for space heating applications be sloped at the degree of latitude, plus 10° to 15°. By having the collectors at this slope, the incident radiation is maximized during the months in which there is a space heating requirement, however, there are other factors to consider. Unless the collectors can be supported on a sloped roof of this angle, a collector support rack must be built.

Figure 3. Solar Radiation monthly comparison for collector slopes

Figure 3 graphs the incident radiation on a horizontal, vertical and a 60° sloped surface in Ottawa and illustrates that a vertical collector performs close to that of a sloped collector without any ground reflectance. When ground reflectance is included, a vertical wall will produce from 15% to 30% more heat than a collector at a 60 degree angle. For heating of buildings in northern latitudes, a vertical wall is therefore the preferred surface for mounting solar collectors.

There are other advantages to vertically mounted collectors versus sloped collectors.

• Incident radiation during the summer months is greatly reduced on a vertical surface, thus reducing heat gain during these no-load periods.
• The structural costs for wall-mounted systems are low.
• Duct losses for wall-mounted fans are non existent.
• Snow build-up is not a problem
• Vertical panels rarely add wind loads to the building.
• Installation costs are lower

Solar Heating Efficiency

The efficiency of a solar collector is highest when the temperature of the air entering the solar panel equals ambient temperature. This occurs with the transpired panel since outside air always enters the system.

In space heating designs, building return air enters a solar panel to be heated above room temperature. On cold, overcast days, there may be insufficient energy to achieve this, whereas, when heating ambient air, any heat gain, whether it be a rise of two degrees or twenty degrees, is useful energy

Performance Example

Using the solar efficiency curve in figure 4, the solar performance of heating fresh air can be compared to conventional solar heating systems.

Assume:
Plant air temperature: 20°C
Outside temperature: -10°C
Solar radiation: 1000 W/m2
Recirculating plant air through solar panels: X-axis intercept (20-(-10)) /1000 = 0.03

Therefore, efficiency is 30% from graph

Drawing ventilation (outside) air through solar panels: X-axis intercept (-10-(-10)) /1000 = 0

Therefore, efficiency is 60% from graph

Performance of a system heating ambient air can be double that of other solar heating designs.

Figure 4. Typical solar efficiency curve for heating fresh air versus heating room air