Shabbat: Solar Heating and the “Dud Shemesh”
Although cooking on Shabbat in the normal manner is generally forbidden, cooking by the direct heat of the sun is often permitted. The rabbis permitted making direct use of the sun's heat because cooking in this way does not resemble the normal manner of cooking. As such, one is permitted to fry an egg outdoors in the heat of the sun on Shabbat. However, cooking through indirect solar heat is forbidden. Therefore, one would not be permitted to leave a frying pan out in the hot sun in order for it to heat up, and then use the heated pan to fry an egg indoors. The rabbis prohibited doing so lest onlookers be led to believe that the pan was heated in the normal weekday manner -- in violation of Shabbat -- and not by means of the sun.
A more common and practical application of solar heating relates to the “dud shemesh,” - the residential solar water heaters that are common in Israel. There is much discussion on whether it is permitted to make use of the water that is heated in such boilers on Shabbat. In order to properly understand the arguments on each side of the issue, it is important to understand how these solar water heaters work.
In most models, cold water enters the hot water tank through a hose attached at its bottom. Inside the tank there is special piping which includes a heating element that traps heat generated by the sun in order to heat the water as it passes through the piping. When the water inside the piping reaches the desired temperature it is sent to the top of the tank to be used. This cycle continues until all the water in the tank has been heated to the desired temperature. The hottest water is that which is at the top of the tank and from there it flows throughout the house whenever a hot water faucet is turned on.
There are two primary questions concerning the permissibility of using these solar water heaters on Shabbat. One is whether the piping that traps the sun's heat and then heats the cold water that passes through it is considered to be a direct or indirect use of the sun's heat. The other question is whether the cold water that enters the tank after hot water is removed (from using the faucets) is being "cooked" by the hot water that is already there.
A number of authorities permit the use of these solar heaters. This is because even though it may appear that the water is being heated by a secondary device (the piping), and not by the sun itself, it is argued that since the pipes merely store the heat produced by the sun and do not inherently heat the water, the heating process is still deemed to be direct use of the sun. Rav Shlomo Zalman Auerbach is also among those who argued that the mechanics of a solar water heater is considered to be "direct solar heat" and permitted their use on Shabbat. It must be noted, however, that if there is any electricity involved in heating the water then it would not be permitted to use the water. Therefore, when using hot water from the tank on Shabbat one must be sure that it was heated by solar heat only (e.g. “the button” must be off).
Other authorities disagree and rule that the mechanics of a solar water heater cannot be categorized as a process of direct solar heat. They argue that regardless of whether the piping actually heats the water, it is a vital component in heating the water nonetheless. Indeed, if one were to simply place the water tank in the sun without the assistance of the special piping, the water would never get as hot as it does. As such, they rule that solar water heaters work with indirect solar heat. Therefore, they may not be used on Shabbat. Furthermore, these authorities argue that even if the manner in which the solar heaters operate can be categorized as direct solar heat, one would still need to justify permitting the use of the cold water that enters the tank every time hot water is removed. This is because the cold water that enters the tank is immediately heated by the hot water that is already there! Therefore, the cold water is essentially being heated through a form of "indirect" solar heating, which is forbidden.
Nevertheless, a number of authorities, including Rav Ovadia Yosef, permit the use of these solar water heaters for a completely different consideration. This is because, as mentioned, the prohibition against making indirect use of the sun’s heat on Shabbat is a rabbinic enactment. There is a general rule that a rabbinically prohibited action, performed on Shabbat in an indirect manner, is permissible. In this case, any possible forbidden actions (using the hot water that was heated indirectly by the sun and the heating of the cold water as it enters the tank) is rabbinic in nature and is being done indirectly (by simply turning on the tap) and is therefore permitted. This situation is known as a “pesik reisha, al yedei gramma, b’issur derabanon” (an inevitable action, caused by secondary force, in a rabbinical prohibition). An action will almost always be permissible when the issue is one of “pesik reisha, al yedei gramma, b’issur derabanon”.
One can also argue that the use of these solar water heaters is one of “pesik reisha d’lo nicha lei” which is when one has no interest or need for an automatically resulting melacha. Although one may not perform a pesik reisha d’lo nicha lei when dealing with a possible biblical prohibition, many authorities permit pesik reisha d’lo nicha lei when the issue revolves around a rabbinical prohibition. The reason why the use of the solar water heaters can be considered a situation of pesik reisha d’lo nicha lei is because the amount of hot water in the tank is surely enough to provide for all of one’s Shabbat needs, such as washing one’s hands and face or washing the dishes, without the need for additional water to be added on Shabbat itself. As such, one has no true need or interest in the cold water that inevitably enters the tank and is heated up by the water that is currently there.
 Shabbat 38b-39a,Rashi; OC 318:3.
 Yabia Omer 4:34; See: http://en.wikipedia.org/wiki/Solar_hot_water#Types.
 Tzitz Eliezer 7:19; Yabia Omer 4:34; Har Tzvi, OC 188.
 Shemirat Shabbat Kehilchata 1:45 footnote 127.
 Minchat Yitzchak 4:44.
 Har Tzvi, OC 188; Or Yitzchak 164.
 This issue is discussed in the Yabia Omer 4:34 cited above.
 For a list of authorities on both sides of the issue, see Rivevot Ephraim 3:263.