Ultra-lightweight, high-performance, foldable: advances in space photovoltaics

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.

Interview by Anaïs Marechal