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The math calculations are summed up in the Solar Oven Spreadsheet. It requires a Java-compliant web browser and will open in a new browser window. It takes a moment to load the page. It works better in Internet Explorer than in Netscape. You can download a copy of this spreadsheet and the Java files that make it work in this zipped file. It is possible to use this same spreadsheet, online or downloaded, to analyze other types of solar ovens, even those that do not retain heat. Some of the calculations on the spreadsheet require some explanation, given in detail on this page.
There are a number of possible design variations for the HRSO. This second HRSO spreadshee explains the design of a triangular shaped
There is also a general purpose solar oven calculator (a simplified version of the above spreadsheet), which does not take into account heat retention. A zipped download of the solar oven calculator is available here. And a new (experimental) calculator for Solar Water Distillation.
Using the spreadsheet, you can change the various specifications for the Heat Retention Solar Oven, or any solar oven, and immediately see whether you would obtain more energy for the oven, or less.
For example, change the width or length or height fields and then look at field (f27) to see the change in Net Energy. Notice that a shorter solar oven will retain more net energy. If you raise the height value too high, the net energy will be negative. This does not necessarily mean that the oven will not work. You must adjust other values to see if you can obtain a positive net energy.
Increasing the size of the Reflector will increase net energy. The spreadsheet sets the reflector size to the width times one meter, plus the length times one meter. In other words, each of the four sides of the solar oven has a 1/2 meter reflector. You can click on the Reflector field (b13) and backspace to delete the current formula (it will be returned to the default when the page is refreshed). Then enter any number you'd like to test.
Decreasing the cooking temperature (c14) will increase net energy because heat loss depends partly on the temperature difference between the interior and exterior of the oven. Will the oven reach 450 degrees F? Enter 450 into field (c14) and see if the net energy is positive or negative. A solar oven design that fails to cook at 450 F may still cook at 300 F.
The Heat Loss section of the spreadsheet looks at loss of heat energy through the walls, floor, and top of the solar oven. Since this oven is meant to be covered whenever there is insufficient sunlight, there are differing amounts of heat loss depending on whether it is covered or uncovered. One of the main obstacles to any solar oven design is minimizing heat loss through the transparent window on top of the oven. This design uses three panes of glass with a 2-inch space between each pane and an estimated R-value of 3.23.
Lines 22 thru 25 calculate the amount of energy input from the sun. Line 23 takes the average daily solar irradiation (sun energy) and discounts it to 80% because the solar oven will not function effectively in the early or late daylight hours. The oven is best kept covered during those times and therefore about 20% of the daily energy from the sun is not used. If the oven were uncovered, it would lose more heat than it would gain during those times. (Line 23 does not effect any of the other lines in the spreadsheet.) Line 25 takes the hourly solar irradiation (watts per sq. meter) and multiplies it by the number of uncovered hours, then discounts it because the glass window does not let 100% of the sunlight into the oven.
Line 27 tells you if the solar oven produces more energy than it uses.
What are some Additional Possibilities? Read On....
Disclaimer: The drawings, procedures, and words on this site are for information purposes only. No claims are expressed or implied as to the safety, usefulness, or accuracy of this information. This site does not contain recommendations or actual plans for building a Heat Retention Solar Oven. This particular solar oven design is theoretical and experimental.