Chapter 8 LO4: Demonstrate A Practical Understanding Of The Operation Of Tbe Plate And Evacuated Tube Solar Thermal Collectors

Question 1

Describe the basic operation of a flat plate collector

Answer 1

Cold water is pumped into the collector at low level. Here it enters and travels up heat absorbing riser tubes. Water enters at a low level and rises up the collector to correlate with natural convection currents. Which also rise upwards. The heat absorbing riser tubes are connected to an absorber plate, either on top or below, it has a special black paint applied which maximised the systems absorption of solar radiation. Under this absorber plate is insulation which reduces the heat losses from the system through conducting and convection. There is a glazing sheet cover on top of the panel. This glazing sheet is designed to :

1. Increase the solar radiant on transmitted tn right it to the absorber plate.
2. Reduce as far as possible heat losses back through the glazing sheet cover through conduction and convection.
3. Protect the system from rain and weather conditions.

Question 2

What happens in the heat absorbing pipe?

Answer 2

As the water rises up the heat absorbing riser pipe it absorbs energy from the sun and absorber plate. This increases the temperature of the heat transfer fluid used. When this fluid reaches the top, it leaves the panel to be pumped to the hot water tank within the dwelling. Depending on the climate at the location, the heat transfer fluid can be either water or an antifreeze fluid. In this indirect loop system a heat exchanger would transfer the energy from the anti-freeze to the heat water tank.

Question 3

State the advantages of the flat plate collector.

Answer 3

1. It is the cheapest collector available which is its main benefit.
2. It is suited to colder environments where snowfall is likely. This is because the snow can be melted due to conduction and convection heat losses from the panel.
3. The efficiency of converting solar energy into thermal energy is good at approximately 80%.

Question 4

State the disadvantages of the flat plate collector.

Answer 4

1. Even with the use of inflation to maximise the heat losses, they do exist, through conduction and convection within the flat plate collector.
2. In higher temperatures the collector efficiency drops considerably especially over 30 degrees.
3. Installation required at least 2 people.
4. As the collector surface is flat it will only be facing the sun directly (angle of incidence = 0) once per day.
5. The weight of the panel filled with the heat transfer fluid can impose considerable loads on the roof surface of the structure.

Question 5

Q4

Answer 5

EX 8.4 Textbook

Question 6

Describe the operation of the evacuated tube

Answer 6

The operation of the evacuated tube is more complex than that of a flat plate collector. The operation of a single evacuated tube is as follows:

1) A double walled glass circular cylinder called the evacuated tube contains an absorber plate connected to a copper heat pipe as shown in fig 8.20. This double walled evacuated tube has a vacuum between the 2 glass walls, which reduces heat losses via conduction and convection to almost 0.

2) The absorber plate is attached to the copper heat pipe. As solar radiation strikes the absorber plate, energy is transferred to the copper heat pipe.

3) The copper heat pipe has an enclosed cavity which is partly filled with a heat transfer fluid in liquid form. This fluid in NI is a mixture of water and glyscenol to reduce its freezing temperature.

4) Then the absorber plate transfers energy to the copper heat pipe this heat transfer fluid is heated at the lower end of the copper heat pipe, heat transfer fluid increases in temperature evaporating into a gas which rises up the tube because of convection

5) The gas reaches the top of the copper heat pipe, which has the water circulating through it.

6) The high temperature vapour loses energy to the colder water in the copper manifold heat exchanger. The vapour condensed back into a liquid when sufficient energy is transferred to the water.

7) The heat transfer fluid now in liquid form returns to the bottom of the copper heat pipe under the action of gravity.

8) The cycle, incorporations steps 1 to 7, continues provided solar radiation is incident upon the evacuated tube.

Question 7

Advantages of an evacuated tube

Answer 7

1. Its circular geometry results in the tube facing perpendicular to the sun for the most of the day.
2. Heat losses through conduction and convection are almost 0.
3. There is less loading onto the roof and support structure as the system is not filled entirely with a heat transfer fluid.
4. It has higher efficiencies, of as much as 90%, that can be maintained at very high temperatures.
5. They can typically be installed by 1 person.
6. As they can be sorted vertically, transportation costs can be minimised.

Question 8

Disadvantages of an evacuated tube

Answer 8

1. It has a higher cost compared to a flat plate collector.
2. Where snowfall is possible, the evacuated tube is not able to melt the snow as easily due to lower heat losses through conduction and convection.

Question 9

Q5

Answer 9

EX 8.4