Here Comes The Sun

Here Comes the Sun

I thought this project would be interesting because I was amazed at how hot surfaces can get during the winter. My dad inspired me to do this project because one day we were outside and I was up against the wall and it was very warm, but the air temperature was much colder. I wondered how much temperature difference there was. This project is trying to examine optimal conditions for passive solar heating by studying the effect of solar radiation on temperature of different surfaces. People who could benefit from this project include anyone who is trying to find the most efficient way to heat their home or any other space.

My project included some background research about the 3 heat transfer methods, radiation, convection, conduction.  Passive solar energy is collected by exposing a surface to radiation from the sun. The energy is then conducted into a thermal mass. This heat is released by both convection and radiation.

My materials for my experiments are as follows: canvases, instant concrete, some tape, a wooden frame, paint, a data logger, thermocouple wires, a pail, a few trees and the sun!

Experiment 1: Colour and Angle Effects

This experiment includes collection of temperature data from different colours and surfaces in my backyard throughout the day and night.

I painted canvases the following colours: Black, Blue, Red and White.

I used instant concrete to fill the back of canvases to store heat and even out temperature fluctuations. I measured the angle of the sun at noon on Feb 20th. This angle was 25°.  I mounted my canvases onto a wooden frame that had 2 angles: Vertical and 25°.

I set the data logger to record every 2 minutes. I did my experiment from February 23rd to March 7th. I downloaded my data and turned them into charts.

Experiment 2: Thermal Mass Effects

I mounted seven thermocouple wires on a wooden dowel. Each thermocouple was one inch apart. I filled a bucket with concrete eight inches deep and placed the thermocouples in the centre. I painted the surface of the concrete black. I placed the bucket inside a plastic tub full of insulating hemp shavings. I placed the bucket on the south side of my house from April 18th to 23rd and recorded the temperature data.

Experiment 1: Color and Angle Effects

The thermocouple placed adjacent to the wall had temperatures that fluctuated due to the placement directly in air.  To store the heat from passive solar radiation, a surface needs sufficient mass to absorb heat energy.  Concrete tiles provide sufficient thermal mass for short term storage of heat.  To store passive solar heat from day into the nighttime, you would need much more thermal mass.

My experiment confirmed that to receive maximum heat transfer from solar radiation, you need to have a clear view of the sun! My backyard had a garden shed, a trampoline and two trees. When examining the data, one can see dips in the temperature. These dips correspond to shadows from these objects. Note the dips in temperature through the day on Figure 2.

The black tiles experienced the highest temperature rise when exposed to solar radiation.  The white tiles had the lowest temperature rise.

This is due to the different absorption characteristics of these colours.

White light from the sun is composed of a spectrum of radiation.  The black colour absorbs most of these wavelengths and thus is perceived to be black by an observer.  The white tile reflects most of the light and therefore is perceived to be white.  If the goal is to absorb solar radiation for passive heating, it is important that the surface exposed to sunlight be dark in colour.  See Figure 3.

Surfaces that are perpendicular to the sun’s rays have the maximum amount of radiation absorbed.  Any surface that is at a different angle will have reduced exposure proportional to the cosine of the angle.  When examining maximum temperatures of the black tiles, the temperature above ambient for the vertical tile was reduced by very nearly the cosine of the angle (25°). See Figure 4.

Experiment 2: Deep Thermal Mass

In any solid material, heat always conducts from high temperature to low temperature. When I did my second test with a pail and larger thermal mass, I found that there are 3 main phases: absorbing heat, two-directional flow and releasing heat.

When the surface is exposed to radiant heat, the top level of the concrete block increases in temperature. This heat is conducted in one direction:  from the surface into the concrete mass.  The gradient of heat is highest at the surface (1”) to the lowest (7”).

As the sun starts to set, the incoming radiant heat load becomes less than the heat loss to the atmosphere. The temperature of the surface declines rapidly as heat is transferred in two directions: both back to the atmosphere and deeper into the thermal mass. As two-directional heat transfer progresses, the highest temperature recorded in the concrete mass starts to migrate from the surface into deeper levels until the concrete mass becomes a near-uniform temperature. In Figure 5, this temperature is approximately 29°C. At this point, the mass resumes single-direction heat transfer in reverse. Heat is released from the thermal mass to the atmosphere.

The following conclusions are important factors for effective passive solar heating:

The absorbing surface needs a clear view of the sun.

Dark colours absorb incoming radiation more effectively. Black canvas had the highest temperature rise.

The amount of energy absorbed can be maximized by positioning the angle of the surface perpendicular to the sun's rays.

To store passive heat, you need more thermal mass than the 8” depth of concrete I used in my experiment.

At the 7” depth I still measured temperature fluctuations of up to 18°C.

If I had more thermal mass would have stored more heat and the temperature wouldn't have fluctuated as much.

I think I could have used more variation in days. For example, examining colder days.  It would be interesting to see the effect of more thermal mass (20 inches deep). If I moved my stand inside and examined how much hotter it could get, this would make the conditions different because when the sun starts to shine on the surface it will increase faster because the air temperature is already hotter.

Thanks to my wonderful dad for letting me use his data logger and thermocouple wires!

Incropera, F; DeWitt, D. 1996. Fundamentals of Heat and Mass Transfer. Wiley and Sons.