Workshop 5     CONCENTRATORS

Unlike other animals humans have developed an addiction to energy in a concentrated form. Ever since the discovery of fire hundreds of thousands of years ago people have relied on wood to heat their caves and cook their food. Coal and oil have replaced wood as the energy concentrate of choice in the last century, but these resources are limited and their combustion products are damaging our environment. Unfortunately conventional power plants use energy concentrates like coal or oil or nuclear fission to supply the heat needed to produce super heated steam (1000 F) and generate electricity.

Solar thermal power plants concentrate the sun's energy to produce the same kind of super heated steam made in fossil fuel plants. The two basic kinds of concentrators are the parabolic dish and the parabolic trough. A parabolic dish concentrated light on a point and a parabolic trough concentrates light on a line. Dishes or heliodynes achieve higher temperatures than troughs but they're less efficient, more difficult to construct and require a precision set of dual tracking devices.

PARABOLIC DISH

Troughs are more popular than dishes in solar power plants because they are less expensive, easier to build and require less maintenance. In this online class we'll build a simple parabolic trough and measure it's heat collecting ability.

Parabolic Trough Power Plant

BUILD A PARABOLIC TROUGH

To build a parabolic trough or parabolic dish you'll first need to sketch a quadratic curve on a board or piece of cardboard. You'll need to decide on the size, shape and focal point of the dish. The following equation will help you locate the focal point "p" :  Y=X²/4p
For this exercise I'd like everyone to use 2" for the value of p and one inch increments for the values of X. Find a piece of plywood or piece of cardboard 16"x 8" and mark off a grid pattern in one inch increments. Click on the graphic on the top of the page for additional guidelines.

EXPERIMENT 1  MAXIMUM TEMPERATURE

A inexpensive meat thermometer could be used for this experiment, but a quality digital thermometer will give you more accurate readings.. Everyone should take measurements on a clear day so that we all have something standard to compare our findings. To be scientific record the outside temperature, location, time of day, and wind velocity. These factors will all have an influence on our temperature readings. The flow pipe at the focal point of the parabolic trough is where the solar heat will be concentrated. Aim your trough directly into the sun, insert your digital thermometer into one end of the flow pipe and measure the temperature. Tilt the trough at various angles and  take additional readings.

Use the following format when posting your experimental results:

LOCATION.....................................
DATE...............................................
TIME OF DAY..................................
AMBIENT TEMPERATURE.............
WIND VELOCITY
TYPE OF GLAZING..........................
TYPE OF REFLECTING SURFACE
MAXIMUM TEMPERATURE RECORDED etc.

EXPERIMENT 2

Repeat experiment 1 with a thin sheet of plastic stretched over the top of the trough. Record your data and post this it on the CONCENTRATOR FORUM. How does the covered trough compare with the uncovered trough. Do you think different glazing materials would yield different results? Please present your data in the format suggested above. 

EXPERIMENT 3 HEAT
Solar applications that generate high temperatures have value, but the quantity of heat they generate is also important. To assign a value to our project we will need to calculate the quantity of heat our trough collects over a period of time. 1000 BTU's of heat has a value of about $0.01. 

You will need to connect a hose or plastic tube to one end of the flow tube. Bend the tube upright and place a funnel in it. Now get a quart of water with a known temperature (the colder the better) Have two containers that can catch and transfer the water. Pour the quart of water slowly into the funnel and collect it when it exits the flow tube at the other end of the trough. Remove the container of  collected water and replace it with an empty container. Now pour the heated water back through the funnel and repeat this process for ten minutes. Now measure the temperature of the heated water.

Since a quart of water weighs about 2 pounds the heat collected will be 2 pounds times the temperature difference between the initial temperature of the water and the ending temperature of the water. Q = (t2-t1) x mass of water.

A temperature difference of 10 degrees F means that you collected 20 BTU's of heat. In one hour that would be 120 BTU's. This would mean that your trough is producing heat at a value of $.0012 per hour. Please submit the heat collected data for EXPERIMENT 3, CONCENTRATOR  FORUM in the standard format. 

You may also suggest ways of improving the accuracy of your data collection method and also suggest methods of improving this kind of apparatus. If you would like to publish an article to this page or any other page with links back to your own page you may submit your article to:

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