# John I. Yellott

John Ingle Yellott (October 25, 1908 – December 30, 1986) was an American engineer recognized as a pioneer in passive solar energy, and an inventor with many patents to his credit. In his honor the American Society of Mechanical Engineers (ASME) Solar Division confers a biannual "John I. Yellott Award" which "recognizes ASME members who have demonstrated sustained leadership within the Solar Energy Division, have a reputation for performing high-quality solar energy research and have made significant contributions to solar engineering through education, state or federal government service or in the private sector."

## Early life, education, academia, and war service

John Ingle Yellott was born in Bel Air, Maryland, the son of the Reverend Dr. John I. (1873-1935) and Mildred Walker Nelson Yellott (1876-1954). His grandfather was John I. Yellott. He was educated at Bel Air High School and Episcopal High School in Alexandria, Virginia, and then studied Mechanical Engineering at Johns Hopkins University, Baltimore, receiving his M.E. in 1931 and his M.M.E. with distinction in 1933.
Yellott embarked on a career in academia at the Stephens Institute of Technology, Hoboken, and at Illinois Institute of Technology, where he was chairman of the Department of Mechanical Engineering from 1940 to 1943 and director of the Institute of Gas Technology from 1943 to 1945. During World War II he was assigned to the Manhattan Project to work on the development of atomic weapons as a consultant to the Metallurgical Laboratory of the University of Chicago.

## Changing fields

After the War Yellott served as Director of Research, Locomotive Development Committee, Bituminous Coal Research from 1945 until 1955. In 1955 at age 47 after having already achieved recognition in steam, coal, gas, and nuclear energy, he switched fields—to passive solar energy. Yellott said the change resulted from "the realization that the entire world, and the United States in particular, was running out of fluid fuels, and that we must find a way to gain economic access to the limitless energy of the sun." In a 1974 interview with Popular Science Magazine Yellott said he anticipated soon "[t]he inexorable forces of economics will bring about the age of solar energy." He organized the 1957 Living with the Sun competition, a seminal event in the history of the solar house.

## Industrial consulting and return to academia

In June 1958 Yellott founded John Yellott Engineering Laboratories, and the Yellott Solar Energy Laboratory, in Phoenix, Arizona. He became an industrial consultant, with a primary focus on reflective glazing. He served as Headmaster and then Director of Development for Phoenix Country Day School, and taught environmental control systems at the College of Architecture at Arizona State University. As the first Chairman of the ASME Solar Energy Applications Group (later Solar Energy Division) he was in a position of leadership "that was critical to the official 'rediscovery' of solar energy following the 1973 oil crisis". Soon after the oil crisis, Arizona State's College of Architecture instituted a solar program and chose Yellott as its head; he continued to teach there until his retirement at age 70.

## Solar engineering

Yellott's early interest, the source of much consulting work, was on the transparency and transmissivity of glass. In 1960s he helped to develop the solar heat gain factor (SHGF) method of calculating the passive thermal role of glass, now the standard method accepted by the American Society of Heating, Refrigerating and Air-Conditioning Engineers. He served as a consultant to major industrial concerns such as Corning Glass, PPG Industries, Libbey-Owen-Ford Glass company, Shatterprufe Glass Company of Port Elizabeth, South Africa; and Pilkington Brothers Glass Company of Great Britain. One of Yellott's clients was Northrup, Inc. whose founder Leonard L. Northrup Jr. was introduced to solar technology by Yellott, and whose company went on to develop some of the first solar air conditioning systems and heliostats, under Yellott's on-going advice.
According to his Illinois Institute of Technology Hall of Fame entry, Yellott's work resulted in more than fifty patents, such as an electric locomotive with a coal fired gas turbine using pulverized coal directly injected into a combustion chamber; a film-type solar water heater and a transparent cover to heat swimming pools by trapping solar radiation Yellott's pool cover idea remains in commercial use to this day. He also contributed his solar expertise to civic projects such as the landmark Carefree Sundial in Carefree, Arizona.
Yellott pioneered passive solar cooling as a field parallel to solar heating, with particular attention to water spray and controlled evaporation. As with any innovator, not all experiments met with success.
Yellott's collaboration with solar entrepreneur Harold Hay on developing an evaporative solar system called a roofpond, which Yellott considered "the simplest system which can accomplish both heating and cooling with the same equipment,” proved to be not so simple. The experiments at Yellott's laboratory encountered obstacles such as the unexpected "healthy growth of 'wrigglers' within plastic-enclosed water" from the city mains; then "a pair of nesting birds was attracted to the comfort of a projecting portion of the roofpond. . . ." The researchers eliminated the unwelcome 'wrigglers' as well as a prolific growth of algae with chemical treatment; they considered the “cheerfulness of the birds a pleasant relief from the drudgery of data-collection,” but recommended “preventive measures for those not wanting such company.”
Yellott's and Hay's experiments did prove the roofpond concept technically feasible, later confirmed with expanded testing on dormitory roofs at Trinity University in Texas. Nonetheless, in the words of solar scientist Kenneth Haggard of the San Luis Obsispo Solar Group, implementation of the idea of maintaining a large puddle of water on one's roof to cool the interior "awaits the next period of blossoming of passive solar architecture." (Hay defended roofponds, acknowledging that while a roofpond design error proved "highly expensive to a young architect and his client," nonetheless there is an "unnecessary fear of having bodies of water overhead." Hay also noted that a review of all roofpond installations in the United States concluded the roofpond "outperforms any other single passive system in both heating and cooling modes" and opined "It may be DOE's best kept-secret buried under hundreds of reports.")
Yellott's achievements particularly in the area of glazing and solar radiation capture have stood the test of time. According to Dr. Jeffrey Cook, College of Architecture and Design, Arizona State University, writing the introduction to the Passive Solar Journal's John I. Yellott Memorial issue, "he made major contributions in areas of basic research, instrumentation, analytic methods, applications, collaboration, education, and demonstration." Cook also wrote: "[h]e published no definitive book," but "[a]rticles bearing Yellott's name will remain classics in solar energy literature;" he "developed no distinctive demonstration, no outrageous solar machine, and no landmark building or award winning engineering system," yet "as a consultant he influenced such major architectural feats as the St. Louis Arch and the new Hong Kong and Shanghai Bank;” his analysis “contributed to the industry development of architectural glasses . . . that have gained worldwide use.” His pervasive influence in solar science is largely found in the ongoing work of others, as “he was a mentor to architects, inventors, and students.” "Thus,” Cook concluded, “as gentle and persuasive as the sun, Yellott touched many people and places.".
Yellott died of a heart attack in Phoenix in 1986, survived by his (second) wife Barbara, a son and a daughter, two stepsons and six grandchildren.