Daniel Lin­cot. The pho­to­volta­ic effect was dis­cov­ered by Edmond Bec­quer­el in 1839. Sil­i­con solar cells were invent­ed before the Sec­ond World War by Rus­sell Ohl, and the first patent was filed in 1941¹. In 1954, the first effi­cient solar cell was man­u­fac­tured, achiev­ing an effi­cien­cy of 6%. The Unit­ed States, which was look­ing to sup­ply satel­lites with ener­gy, imme­di­ate­ly began pro­duc­ing cells for satel­lites. In 1958, Van­guard 1 was the first satel­lite sent into space with solar cells. The cost of pho­to­voltaics was extreme­ly high at the time [Editor’s note: In the 1950s, one watt of solar pho­to­volta­ic capac­i­ty cost £1,865, adjust­ed for infla­tion and 2019 prices. Com­pared to the price in 1956, a solar mod­ule today would cost £596,800², but afford­able com­pared to the cost of launch­ing a satel­lite into space. The increas­ing pro­duc­tion of solar cells for space appli­ca­tions has led to a decline in costs, which con­tin­ues today. With­out the use of pho­to­voltaics in space, the tech­nol­o­gy would prob­a­bly not have devel­oped as quickly.
Loris Ibar­rart. Satel­lites are autonomous objects, par­tic­u­lar­ly from an ener­gy per­spec­tive. To car­ry out their mis­sions, whether for telecom­mu­ni­ca­tions, mil­i­tary pur­pos­es, or space or Earth obser­va­tion, they must be able to com­mu­ni­cate and to sur­vive. For this they need ener­gy, to send and receive infor­ma­tion from Earth, keep equip­ment at the right tem­per­a­ture and main­tain alti­tude. Final­ly, the mis­sion itself also requires ener­gy. While Earth obser­va­tion mis­sions do not con­sume much ener­gy, telecom­mu­ni­ca­tions do. Ini­tial­ly, a bat­tery was placed on board the satel­lite. Its sur­vival capac­i­ty was only a few weeks. Space indus­try play­ers there­fore thought of using the only resource avail­able in space: the Sun.
DL. The first satel­lite launched into space in 1957, Sput­nik, was only able to com­mu­ni­cate with Earth for a few weeks! It then remained in orbit around Earth, with no means of communication.
LI. From the out­set, there was an idea to use pho­to­volta­ic pan­els on Earth, but the tech­nol­o­gy was only viable for the space industry.
DL. From the 1970s onwards, the first ter­res­tri­al appli­ca­tions emerged: equip­ment for light­hous­es and bea­cons in iso­lat­ed areas, means of com­mu­ni­ca­tion in inac­ces­si­ble loca­tions, and cathod­ic pro­tec­tion for oil pipelines to lim­it oxi­da­tion. These very spe­cif­ic uses jus­ti­fied the high prices. Then the cost of cells began to fall, par­tic­u­lar­ly fol­low­ing the 1973 oil cri­sis, which led to an increase in pro­duc­tion and there­fore a reduc­tion in costs due to economies of scale.
LI. The require­ments of the space indus­try remain the same: to pro­duce as much ener­gy as pos­si­ble on board for the small­est pos­si­ble mass and vol­ume. Most satel­lites orbit­ing the Earth are equipped with pho­to­volta­ic pan­els. Telecom­mu­ni­ca­tions satel­lites have the high­est pro­duc­tion capac­i­ty, which can reach up to around 30 kW, or a solar cell sur­face area of approx­i­mate­ly 100 m². Only a few mis­sions can­not rely entire­ly on solar pow­er: probes sent into deep space and rovers [Editor’s note: “astro­mo­biles”, vehi­cles designed to explore the sur­face of a celes­tial body]. They car­ry a nuclear core, a kind of radioiso­tope bat­tery. This ener­gy source is more expen­sive and restric­tive than photovoltaics.
DL. Pho­to­voltaics have come a long way, par­tic­u­lar­ly in terms of effi­cien­cy, and remain the most effi­cient source of space ener­gy. The effi­cien­cy of the spe­cial cells used (mul­ti-junc­tion) can reach 35%. A record effi­cien­cy of 47% has been achieved in the lab­o­ra­to­ry. In com­par­i­son, the effi­cien­cy of ter­res­tri­al sil­i­con cells is just around 25%.
DL. The the­o­ret­i­cal effi­cien­cy of con­vert­ing pho­tons (light par­ti­cles) into elec­tric­i­ty is 85%. Research is very active in this area, and it is a tremen­dous avenue for progress for ter­res­tri­al appli­ca­tions. Final­ly, space appli­ca­tions increas­ing­ly require very light cells to lim­it launch costs and facil­i­tate deploy­ment. While ter­res­tri­al pho­to­volta­ic pan­els weigh an aver­age of 25 kg per m2, we are work­ing to achieve a weight of 200 g per m2. This is a par­a­digm shift: pho­to­voltaics are becom­ing ultra-light, effi­cient and fold­able. On Earth, this opens up a world of pos­si­bil­i­ties: for exam­ple, we can imag­ine a solar cur­tain that could be deployed on facades, roofs or in the air. These cur­tains could also be tem­porar­i­ly deployed over fields after har­vest to store elec­tric­i­ty. We are work­ing close­ly with the Ile-de-France Pho­to­volta­ic Insti­tute and Ecole Polytechnique’s (IP Paris) inter­face and thin film physics lab­o­ra­to­ry, which spe­cialise in these subjects.
LI. We are also at a turn­ing point for space pho­to­voltaics, with a shift from Earth back to space.
LI. For about 10 years now, we have seen a grow­ing inter­est in ter­res­tri­al tech­nolo­gies in the space indus­try. The rea­son? The rise of satel­lite con­stel­la­tions, for which the space industry’s eco­nom­ic mod­el is no longer com­pat­i­ble. The require­ments here are dif­fer­ent: high pro­duc­tion capac­i­ty, low­er cost and low­er per­for­mance, since redun­dan­cy is ensured by the large num­ber of satel­lites. Today, the cost of cells for space appli­ca­tions is around €300 per watt, com­pared with 10–20 cents for ter­res­tri­al appli­ca­tions. We can imag­ine a com­pro­mise emerg­ing between mass pro­duc­tion for ter­res­tri­al appli­ca­tions and pre­ci­sion work for space appli­ca­tions, even if the future is uncer­tain. The chal­lenge is to mod­i­fy ter­res­tri­al cells so that they can oper­ate for as long as pos­si­ble in space, while mak­ing as few changes as pos­si­ble to exist­ing indus­tri­al chains.
LI. No, the rise of satel­lite con­stel­la­tions will not kill off the tra­di­tion­al space solar pan­el indus­tries. Telecom­mu­ni­ca­tions, obser­va­tion and sci­ence will con­tin­ue to need process­es devel­oped specif­i­cal­ly for space. Man­u­fac­tur­ers have nev­er seen as much demand as they do today.
Geographer and CNRS Research Director
Doctor in Nuclear Physics and Columnist at Polytechnique Insights
Doctor in Nuclear Physics and Columnist at Polytechnique Insights
Scientific Manager of DORN Mission at Institut de Recherche en Astrophysique et Planétologie
weather forecaster at Météo-France
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