A Major Leap Forward in the Art of Solar Cooking

Sun Frost recently built several solar ovens based on the evacuated (Thermos) tube technology commonly used in solar hot water systems. These ovens can achieve temperatures over 600 deg F. Temperature can be easily controlled by covering the collector or turning it away from the sun. The outside of these ovens is close to ambient temperatures even with interior reaching 600 deg F. As a result of the small losses from the walls of the oven high temperatures can be achieved even in partial sunlight. We have built two ovens using this technology. One incorporates a 42” long 5” diameter tube and the second consists of an 18” long 2 ¼” diameter tube.

In the larger oven we’ve cooked a variety of foods from lasagna to cookies. Cooking time are similar to those in a conventional oven.

Sun Frost Solar Cooker – Sterilizer

Sun Frost Tubular Cooker

Sterilize Medical Equipment

Both ovens could also be used for sterilizing medical equipment. Sterilization is achieved when 375 deg F is maintained for 6 minutes.

In developing countries safely disposing of used syringes is often problematic. We have used the smaller tubular sterilizer for melting syringes. The needles are melted into a sterilized plastic matrix, which can be disposed safely.

Last Saturday, JB Straubel picked up his Sun Frost RF16 along with a cabinet. He made the 300 mile trip from Arcata, CA to Menlo Park, CA with one re-charge.

Sun Frost R Pb-X

Sun Frost RFVB with battery storage delivered in Nepal.

Sun Frost RFVB with battery storage delivered in Nigeria.

There are about 2 billion people in the developing world who do not have electricity and in many areas where electricity is available it is only available intermittently. Vaccines for childhood diseases such as polio, measles and diphtheria must be stored at typical refrigerator temperatures, between 2C and 8C (35F and 46F). There are about 4 countries where cases of polio still exists if the children in these countries could be immunized, like small pox, the disease could be eliminated.

There are refrigerators built for remote areas, which are powered by kerosene. However, the units are often unreliable because of poor temperature control, dirty kerosene and problems of maintaining steady supply of fuel. Solar powered refrigerators are gradually replacing kerosene refrigerators. Sun Frost had been building solar powered vaccines refrigerators for the past 15 years and has models in about 50 countries.

The primary components in the refrigerators solar power system are the Photovoltaic panels, charge controller and the battery. The weak link in the system is the batteries which could last between 2 and 8 years. Two years ago “PATH” an NGO supported by the Bill and Melinda Gates foundation put out a request for the development of a battery free refrigerator. They offered to pay for the testing for several companies who submitted what looked like potentially effective designs. PATH offered to pay for testing of a prototype which would be submitted by Sun Frost.

The freezer compartment in a vaccine refrigerator is used for freezing ice packs. The ice packs are placed in an insolated container along with the vaccines allowing the vaccines to be distributed to more remote villages.

Unfortunately, WHO’s initial test procedures were unrealistic. After a year of waiting the test procedures was finalized at the end of May, 2010.

Initially we will be sending a battery free refrigerator for testing which does not have an ice pack freezer.

A chamber in the refrigerator contains 50 pounds of water. The system compressor is directly connected to a Photovoltaic panel. When the sun is out the refrigerator makes ice in the chamber above the refrigerator storage compartment. A sufficient amount of ice is made to keep the refrigerator cold for 5 days in a 32C (90F) environment without the compressor running.

A layer of water always remains between the ice and the vaccine storage area so that vaccines will never be exposed to freezing temperatures. A condition which could ruin the effectiveness of the vaccines. The refrigerator keeps vaccines at an appropriate temperatures from near freezing temperatures to ambient temperatures above 43C (110F) without adjustment. To my knowledge this degree of temperature stability is not available in any other vaccine storage refrigerator.

Installation of the refrigerator in the field will be simple and require only connecting two wires from the PV panel to the refrigerator. In the R Pb-X the temperature is controlled by monitoring water height as the ice freezes. The R Pb-X has no thermostat and the temperature never has to be adjusted in the field. The system should give over 20 years of trouble free service. In locations where utility power is intermittent an AC compressor could be installed on the Sun Frost R Pb-X. When utility power is available the ice storage compartment will be replenished. The Sun Frost R Pb-X will be capable of keeping vaccines cold for at least 5 days with out power. The Sun Frost R Pb-X should prove to be a significant improvement in vaccine storage in remote locations. It will also eliminate the pollution problems of lead acid batteries.

Often over looked is the significant effect a back up system can have on the overall performance of a solar hot water system. In a recent letter sent to Solar Industry Magazine I point out some design considerations.

Several years ago I designed a zero energy home, which I now live in. The considerations made when choosing a back up system are described in a paper presented to ASES.

An area often overlooked when the performance of a solar thermal system is evaluated is how the back up is integrated in the hot water system. Assume that showers are taken early in the morning on a sunny day and the storage tank is depleted of hot water. If the back up system then fills the storage tank with hot water that morning there will be no room for solar produced hot water. If more hot water is not needed until the afternoon, the energy used to heat the water that morning will have been wasted.

This type of management problem can be eliminated if an on demand heater is used as the back up. Unfortunately, neither electric or gas back up are the perfect back up. The disadvantage of a gas back up is that even in the modulating heaters the heat cannot be turned down sufficiently. If, for example, you are taking a shower and your solar tank is at 100ºF you may only need a 10ºF boost in temperature to be comfortable but the on demand gas heater may unnecessarily boost the temperature 25ºF. To remain comfortable the cold water must then be turned up; water and energy are then wasted.

With an on demand electric heater the output can be modulated to a lower level so this problem is eliminated. However, electricity is a more valuable form of energy; three units of gas-produced heat energy must be expended to produce one unit of electrical energy so finding the ideal solution is not clear-cut. In my home I oversized my collectors so they would provide a large percentage of my hot water needs. In this situation, where I often only need a small boost in temperature, I use an on demand electric heater as a back up. To determine the best solution in a particular situation requires some whole system thinking: considering the demand size of the array, storage, and usage patterns etc.