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Solar energy has an obvious problem: the Sun sets. A house that collects heat at noon needs a way to keep it until night, and for cloudy days. Maria Telkes spent much of her career on that problem, and her answer was a cheap salt that melts at about 32 °C, the temperature of a hot day. This post covers her life, the physics of latent heat storage with a worked example, and why her famous house did not work perfectly.
Who was Maria Telkes?
Soma Rédey's study in Hungarian Cultural Studies (2025), based largely on MIT archive documents, records that Mária Telkes was born in Budapest on 12 December 1900 and died there on 2 December 1995. She earned her chemistry and physics diploma at the University of Budapest in 1922 and a PhD in 1924, and arrived in the United States in 1925.
She worked as a research associate at the Cleveland Clinic from 1926 to 1937, then at Westinghouse from 1937 to 1939 on thermoelectric materials. In 1939 she joined MIT's solar energy research group, funded by the Cabot Solar Energy Fund, as its only woman. During the Second World War she developed an inflatable solar still for life rafts that folded to about 60 cubic inches and weighed about one pound.
Why store heat in a melting salt?
When a solid melts, it absorbs heat without getting hotter. The NCERT Class 11 Physics chapter on thermal properties of matter defines this latent heat: the heat needed is Q = mL, where L is the latent heat of fusion. When the liquid freezes again, it gives the same heat back.
Telkes saw that a substance melting at around room-plus temperature could soak up the day's solar heat and release it at night at a steady temperature. In a 1945 report quoted by Rédey she wrote that "Na2SO4 x 10H2O (Glauber salt) may prove the most convenient and least expensive to use, for the heating of houses". Her patent US 2,677,664, granted in 1954, gives its key properties: it gives up heat at a critical temperature of around 90 °F (about 32 °C), and it has a relatively high heat of fusion, 103 Btu per pound.
Worked example: Glauber's salt against water
First convert the patent's units. One Btu is about 1055 J and one pound is about 0.4536 kg.
Latent heat L = 103 × 1055 J ÷ 0.4536 kg ≈ 2.40 × 10^5 J/kg, or about 240 kJ/kg.
Heat stored by 1000 kg of the salt as it melts: Q = mL = 1000 × 2.40 × 10^5 ≈ 2.4 × 10^8 J, or 240 MJ.
Now water. NCERT gives its specific heat as 4186 J/(kg K). Warming 1000 kg of water by 10 °C stores Q = mcΔT = 1000 × 4186 × 10 ≈ 4.2 × 10^7 J, or 42 MJ.
Ratio: 240 ÷ 42 ≈ 5.7.
So, for a 10 °C swing, the salt stores almost six times as much heat per kilogram, and releases it near one fixed temperature. To match it, water would need to warm by 2.40 × 10^5 ÷ 4186 ≈ 57 °C.
Common mistakes: using mcΔT during the melting itself (ΔT is zero then, and Q = mL applies), and mixing Btu per pound with SI units without converting both.
For more practice, our guide to heat of transformation graphs shows how the flat parts of a heating curve represent latent heat. The Fermi Problem Library also has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
What was the Dover Sun House?
MIT's Cabot Solar Energy Fund did not pay for a house built on Telkes's design. According to Rédey, the money came from Amelia Peabody, Eleanor Raymond was the architect, and the house in Dover, Massachusetts, was occupied by the Némethy family from around Christmas 1948. Rédey also quotes a 1945 memo by the group's leader, Hoyt Hottel: "Dr. Telkes's contribution may make a big difference in the outcome of our project."
Why did the salt storage run into trouble?
Telkes's own patents describe the chemistry. The 1954 patent explains that the salt tends to "undercool", staying liquid below its melting point, and that unwanted crystals of the heptahydrate leave excess water. Her answer was a little borax to start crystallisation. A later patent, US 3,986,969 (1976), explains that sodium sulfate decahydrate melts "partly incongruently": some anhydrous sodium sulfate stays undissolved and, being denser, sinks, so less heat is released on freezing. She added clay as a thickener to keep the mixture together.
A 2025 review in Thermal Science sums up the Dover house the same way: it showed that phase-change materials could heat a home, but material stratification limited its success.
What can a student take from how Telkes worked?
Telkes chose a material by its physics, a convenient melting point and a large latent heat, then kept working on its chemistry for decades when it misbehaved. Her patents read like a lab notebook of problems and fixes. That is a useful model: when your answer to a problem fails, write down exactly why, then fix that one thing.
Try it now: using L ≈ 240 kJ/kg, find how much Glauber's salt would store 100 MJ as it melts, and check your units.
