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.
Head of the Universe Sciences programme at CNES
Director of Strategy at CNES
weather forecaster at Météo-France
Research Fellow in Observational Cosmology at Grenoble Laboratory of Subatomic Physics and Cosmology
CNRS Research Director in Astronomy and Astrophysics
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