[MUSIC] We will discuss in these last seconds the various energy storage technologies. The objective is not here to get in detail with this broad topic, but simply to make a quick overview of the various techniques best suited to PV. The storage of electricity is linked to PV system aspect, including issues related to network connection that we show earlier. We present here a comparison on the day between the PV production and demand. Here, PV generation is in excess during the day. The use of storage can convert demand at the beginning of the night, a time when the PV production becomes negligible, while overcoming at least partially of intermittency. In a situation of high penetration of PV, storage can help manage a possible over production, as seen here. Electricity is generally stored from a conversion into another form of energy. Several conversion times are possible, as summarized in this table. One of the main methods is to store mechanically by pumping or compression. It can be stored in thermal form, in particular by phase change. It can also be in electrical or electrochemical form in batteries. Finally, another very attractive form is a chemical storage through fuels, such as hydrogen. The various storage technologies have achieved to date different degrees of maturity. They can be classified roughly in terms of need of investment and level of risk, as shown here. Let's say the more mature are mechanical methods, including hydraulic pump, PHS. Other techniques are close to maturity as molten salts, using CSB, or batteries, NaS, or lithium ion. However, chemical methods, hydrogen, synthetic gas, are in development phase. In terms of need satisfaction, the various storage techniques have very different characteristics, as summarized here. Batteries allow storage from a few hours to several days, with a yield higher than 70%. The PHS allow storage of increased powers or longer type, with comparable yield to the batteries. Only chemical technologies can achieve inter-seasonal storage. The main electrical storage technology, or the cheapest one, is currently the PHS, pumping hydro storage. The principle is summarized here. It is relatively easy, based on two tanks, such as small lakes, separated vertically. When electricity is cheap, it is used to pump water from the bottom reservoir to the top one. On the contrary, during periods of high demands, the system behaves like a conventional hydraulic power. The efficiency in each direction is greater than 90%, giving another yield of about 80%. The stored energy density is relatively low. Then the PHS requires large tanks on, if possible, a large height difference between both. Therefore, available sites are few. Nevertheless, the lower tank may be in some cases the sea. I present here the most important site in the world, located in Virginia, with a capacity of 3 gigawatts, on an elevation difference of 385 meters. The total install capacity of PHS is now 140 gigawatt. This type of storage is ill-suited to decentralize PV. The CAES, Compressed Air Energy Storage, uses electricity to compress air in a confined space, mines, caves, and so on, where it is stored. Conversely, this compressed air is released to activate the compressor of a gas turbine, thus producing electricity. It can store large amounts of energy at low cost. Yield is less than the PHS. As in the PHS, the problem is a small number of available sites. The main site is currently in Germany, it can be charged in 12 hours. The installed capacity in the world is less than 500 MW. If this method can be adapted to artificial dams, it is well suited to decentralize PV. The use of batteries is growing rapidly. They are well suited for decentralized applications, including stand alone. The most used are sodium-sulfide, NaS, and lithium-ion battery. The performances of these two technologies are summarized here. The evolution of the cost of lithium-ion batteries according to the cumulative production is quite similar to that of crystalline silicone modules, as seen here. The drop was almost 22% for each doubling of cumulative production. These batteries are increasingly used in electric vehicles, which explain this price decrease. Future production plant announced by Tesla should generate a new spectacular drop in cost of these batteries. There is here an overall assessment of various electricity storage technologies, largely dominated by hydraulic pump, PHS. Note that the thermal techniques, such as molten salts, are generally suitable for CSP, concentrated solar power. The capital cost varies significantly depending on technology, as shown here, with a significant advantage for mechanical storage, the cost of batteries as a fairly wide spectrum, thank you. [MUSIC]
