Dimiter Tchernev is putting a cool new kink in solar power. When the sun shines, all solar panels produce heat. But this Bulgarian-born inventor, who brought out a solar-powered refrigerator in 1980, is now marketing an entirely new type of solar collector that heats in winter and cools in summer.

His firm, the Zeopower Company in Natick, Mass., is not the only company to coax old Sol to power central air conditioning. There are a few other systems around. The difference, Dr. Tchernev claims, is that his system, on which he holds worlwide patents, is four times as efficient and costs a third as much.

Conventional solar systems require one square foot of collectors for every square foot of floor area to be heated or air conditioned. That makes them expensive. The Tchernev system needs only one-quarter of a square foot of collector per square foot of floor space.

Most solar panels provide only hot water. If you want to add air conditioning, you have to buy a separate absorption air conditioning unit (about yet another expense -- continual maintenance. In addition, there is the significant cost of electricity to drive various pumps and fans.

By contrast, Zeopower's absorption refrigeration system is the only one in the world that is integrated into the collector itself. When the sun is shining , this collector produces hot water during the day and chilled water during the night. It requires no cooling tower or pumps. It has no moving parts to wear out. Dr. Tchernev quips: "We expect it to last forever -- at least."

The collector itself requires no power. A small amount of electricity is needed to drive a fan for a forced-air system and a pump to circulate liquid to and from the collector. But this outlay is minor, he says, when compared with the volume of electricity used to power other solar air-conditioning systems.

Since the Zeopower collector is in active use throughout most of the year, saving on both heating and cooling costs, it pays for itself two or three times faster-than more conven tional solar systems that provide heat only.

In 1976 the US Department of Energy initiated a broad research-and-development program to generate new solar-collector ideas and to bring them through to "relatively proven feasibility." At one point the US Department of Energy (DOE) was supporting about 100 projects under his program.

"Dr. Tchernev's concept was one of the most innovative concepts we received," says Dr. Fred Morse, director of the DOE's Office of Solar Heat Technologies. Many of the other projects, he says, were narrowly focused on some change that would improve performance. "It was the innovation that Professor Tchernev brought to the problem that I think made his work stand out. He is a very innovative, very determined, very persistent person -- all good qualities in coming up with something that is effective and new. I recently saw his collector at a conference and was very impresed with how far he had carried that work."

The simple secret to all the advantages of the Zeopower system is zeolite, a fine-grained yellow mineral about the consistency of powdered cement.

Zeolites have an open molecular structure. When heated by the sun, they expel refrigerant vapor (which in this system is water). When cooled at night, they absorb large amounts of it. thus in the Tchernev system, zeolites perform the same function as the mechanical compressor in an ordinary refrigerator, but without moving parts.

The zeolite collector is amazingly simple. The main portion is like a big, rectangular baking pan about 8 feet long, 2 feet wide, and 2 inches deep. Zeolite is packed into it. A ridged copper lid is laid over the mineral to enhance conduction of heat into it. Black foil goes over that. A heat exchanger, including pipes for circulating the liquid, is placed under the pan. The collector is then hermetically sealed and insulated under annealed glass.

Apart from the collector itself, the Zeopower system is like other solar heating and air-conditioning systems in that it requires a tank in the basement to store the water that circulates through the system. And like every central cooling system, it requires an air-conditioning coil on the outside of the building to eject the heat from the building.

Availability of the key ingredient, zeolite, presents no problem. The mineral results from the reaction of saltwater with volcanic ash and is found in more than 120 deposits in the United States. It is also produced synthetically, but that costs more.

Dr. Tchernev is currently using zeolite from the Arizona desert. He finds this works best in his collectors. He uses about 15 tons a year now. But even should his production demand 100,000 tons a year, that deposit will last perhaps a decade. As his demand increases, he expects industry will be able to manufacture zeolite to his specifications at an economic price.

It was the relentless heat of Texas summers that spurred Dr. Tchernev to think of using solar energy for air conditioning. On a beastly hot day in August 1969, long before the oil embargo hit America in '73, a squirrel expired in the Tchernevs' attic. The inventor, then a professor of electrical engineering at the University of Texas in Austin, ventured up to fetch it, but quickly retreated in the face of wilting heat.

"For some reason my house had a black asphalt tile roof," he recalls. "While it was 100 degrees [F.] outside, it was about 300 in may attic. The next morning I went up again early just as the sun came out and hit the roof. Within minutes the indoor temperature skyrocketed. I couldn't get out of that attic fast enough!"

At that point Dr. Tchernev knew nothing about air conditioning. But the way a flame of gas, creating heat under a refrigerator, could somehow make it cold was something that had always fascinated him. So he began researching air-conditioning systems.

He was studying thermal properties of materials at the time and was using zeolite in some of his experiments. One of his students, either by mistake or out of curiosity, tossed a water-saturated pellet of zeolite into a hot furnace.

"After the smoke of the explosion cleared." Dr. Tchernev says, "I realized that zeolite should be very capable of absorption refrigeration."

"That was the bright light -- that, plus the squirrel in the attic. Those two things put together made me look into the use of zeolites for solar cooling."

He discovered that their special properties were ideal for this purpose. For one thing, properties of most things in nature vary with temperature, but changes are usually gradual. In zeolites, properties change abruptly with temperature.

For example, zeolite is much better at absorbing water at room temperature than any other material. But when heated above a given level, it loses all its water quickly.

Also, to be able to drive a cooling system, ordinary solar panels have to produce water at about 190 degress F. At that high temperature, their efficiency is low. Because zeolites operate at lower temperatures, their efficiency is much higher.

By this point in his career, Dr. Tchernev had demonstrated a gift for creative ideas.He held patents on numerous inventions, mainly in magnetic tape recording, the topic of his thesis for the Bulgarian equivalent of a master's degree in electrical engineering, which he received in 1953 from the Technical University of Sofia. His original work in this field resulted in his being elected (over 180 other applicants) to membership in the Bulgarian Academy of Sciences in 1955, an honor in that country second only to receiving a Nobel Prize.

In 1973 Dr. Tchernev departed from pure research long enough to build a large-scale engineering prototype of an invention, with a view to marketing the solar heating-air conditioning collector himself. "It was obvious the system worked," he says. "The only question was: Can it be mass-produced successfully?"

The timing was excellent. The oil embargo had struck. America had begun to discover solar power. The National Science Foundation gave him research money to pursue his project. The Massachusetts Institute of Technology, where he had earned his doctoral degree in electrical engineering in 1965, invited him to bring his grant with him and continue his work at its Lincoln Laboratory, which he did.

The solar refrigerator was a totally unexpected byproduct. "During the experiment," Dr. Tchernev says, "we couldn't apply heat to the water fast enough , so it kept freezing into ice. Instead of worrying about it, I said, 'let's make it work for us.' So out of that came the idea for the solar refrigerator."

In 1978 Dr. Tchernev took the plunge and left MIT. "I decided if I really believed in what I was doing, I should be able to take the risk and start my own business making it." So he formed the Zeopower Company.

Work went forward at the small plant in Natick (west of Boston) under a US Department of Energy contract on the major project of developing the hot- and cold-water solar collector.

In January 1980 the sun-powered refrigerator made its debut. Today it is being sold globally, mostly for demonstration purposes, as people try them out to see how they work in different climates. There are units in Europe, Africa, the Middle East, and the Orient, with more in Japan than anywhere else. These demonstration units cost $1,695. When they're sold in large quantities, Dr. Tchernev expects to trim the price to $900 or even $600, depending on his volume of sales.

So far interest in Zeopower products and the market for them are almost entirely overseas.Energy costs are so much higher abroad that in many countries solar cooling is the only economically feasible method. It is especially useful in places where there is no electricity or other means of refrigeration.

Dr. Tchernev is discussing a venture with one of the large French companies to manufacture walk-in cold-storage coolers. His solar refrigeration would provide chilled storage for meat, fish, and tropical fruit for the South European and North African Mediterranean markets as well as for most of the equatorial African countries.

The World Health Organization has a world immunization program under way for which it needs vaccines and serums that have to be kept near freezing temperatures. It has searched around the world without finding a way of doing that in the field except to pack them with ice, which lasts only a few days.

Its latest report to UNESCO summarizes the state of the art of solar cooling around the globe. "The way we read it," Dr. Tchernev says, "the conclusion of the report is that our system is the best, most reliable, most practical, and the only one right now operational anywhere."

The UN, he reports, is now trying to get some money to enable Zeopower to modify its solar icebox to meet its particular needs, making it smaller, lighter , and more easily transported.

Under Zeopower's contract with the DOE to develop the solar heating and air conditioning collector, the company is comparing the performance of its panels under different climatic conditions at three sites -- in the hot desert of Tucson, Ariz.; in Golden, Colo., where summer and winter temperatures are both extreme; and at the Zeopower plant in Natick, where summers are mild and winters are quite cold.

"What we are finding," Dr. Tchernev reports, "is that when the sun shines the system works. Therefore, the place with the most sunlight will be the most economical, because our system will operate the maximum number of days in the year."

Right now the solar collectors for the central air conditioning and heating system are being handcrafted in limited numbers at the Natick plant -- at a price that does not lure the average homeowner: $40,000 to $50,000 for a single system that heats, cools, provides heat for domestic hot water and a swimming pool.

Still, Dr. Tchernev says, it is astonishing to learn that there are many communities in the Southwest from Dallas to Los Angeles where home prices start at half a million dollars. "In Palm Springs you can't touch anything below $1 1 /2 million," he says. "So there is a small market right now where we can add our system to a house in that price range."

"We expect to have this kind of market for the next 18 to 24 months. By then we hope to be in mass production," he adds. "Then our prices will go down so that we can start selling to the middle-class residential market. our target price for the system then will be between $12,000 and $20,000, depending on the size of the house. By 1985 my aim is to catch 1 percent of new residential house starts." Sixty-eight units have been sold so far.

In the meantime, federal and state solar tax credits offer help to both homeowners and businessmen. But because the federal credits and tax incentives are greater for the commercial and industrial sector -- under some circumstances a business can end up with no cost at all in the first year -- Dr. Tchernev says it looks as if the commercial market will develop first.

He is working out a rather ingenious system for mass producing his collectors and refrigerator. He has signed an agreement in principle with the Toyo Sash Company Ltd. under which this Tokyo enterprise will finance, build, and put into production a plant in Tucson using the latest automated mass-production techniques. Its initial outpur will be 100,000 collectors a year, with room to expand to 300,000.

When the plant is running smoothly and the target price has been reached, Zeopower will buy the plant from the Japanese.

"In exchange for that," Dr. Tchernev says, "Toyo Sash will be our partner in a joint venture in Japan. It will build a second plant there, manufacturing and selling our collectors throughout the Far East."

Announcement of this plan in the Japanese press brought a flood of inquiries to Zeopower from Japanese companies to see if all business rights had already been sold or if there was still a chance to invest. "I assured them this is only the beginning," Dr. Tchernev said buoyantly.

American Yazaki, with US headquarters in Dallas, is a Japanese company that has high hopes of capturing a large portion of the American market with its conventional solar air-conditioning system. It estimates that by 1990 the market here will be in the $1 billion range. "They've done a beautiful job of engineering the system, reducing the pumps and fans necessary, improving efficiency," Dr. Tchernev says. "But their lithium bromide system requires twice as many collectors as our."

Dr. Tchernev believes this is the only real competition he faces, and he doesn't actually view it as that. "We think we will undercut them in the market ," he says. Yazaki, in fact, is investigating the possibility of incorporating the Zeopower collector into its air-conditioning equipment. "So instead of competing for the market," Dr. Tchernev says, "we may end up working together in some way."

In the meantime Zeopower is negotiating a contract with the State of Arizona to install its air conditioning-heating system on a demonstration house in the Phoenix-Scottsdale area. A demonstration house privately funded by two builders is already going up in Denver.

Overseas, Dr. Tchernev is working on an agreement to build a demonstration unit in one of the Arab oil-producing countries to show that his system can operate in the severe Mideast heat, which reaches 135 degrees F. in the shade in the daytime and falls only to about 95 degrees at night.

If that is successful, he plans to use some extremely well-insulated walls developed by a Scandinavian company which would have numerous applications in the Mideast housing market. Even though oil prices are low there, electricity, he says, is scarce and unreliable. So solar systems are expected to have a future.

Zeopower is also completing discussions with a Swedish company to develop and market a solar milk cooler for the diary industry in third-world countries. "It can use our collector to produce hot water during the day to wash dairy equipment clean. During the night chilled water will be used to cool down fresh milk to room temperature or into refrigeration."

It is too soon for the DOE to give an unqualified endorsement of the Zeopower collector, because it may take another year for its tests to be completed. But Fred Glaski, program manager for Dr. Tchernev's collector, says the technical problems of developing it, which have been "bothersome but not profound, appear to have been solved," and that he expects the collector to achieve the level of performance desired.

The big question now is whether the collector can be made financially feasible through mass production and further simplification of the system to get the cost down.

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