Constant fluid temperature can be achieved using variable fluid flow. For 24 hours operation, back-up fluid heater is needed.

This can be achieved passively as well by adding well insulated thermal mass.

For example use a series of convection water tanks each with an inflow pipe at the top and an outflow pipe at the bottom. The first tanks in pipe receives panel_hot water and its out pipe feeds the in pipe of the next tank. This arrangement out to in repeats until you have enough mass of water in the series to meet your heating demands and then the final tanks out pipe will feed into the panel_cold.

Your building codes should require this type of system to use a transfer fluid with antifreeze for the storage side, a pressure valve and a heat exchanger arrangement to heat the potable water side in isolation. This is due to possibilities for legionnaires disease, frozen burst pipes or spectacular tank steam explosions.

With this arraignment in place you set the temp for the panel output as high as it is safe to do so, you do not want to boil the stored fluid or melt and burst the tanks. The hot fluid entering the first tank displaces the coolest fluid from the first tank into the second, and from the second to the third and so on. When fluid flows out of the final tank it is the coldest from all the tanks and is fed to the pump and thermostat of the system. The potable water is then heated by pumping the hottest water through a heat exchanger to pre-heat a hot regular water tank or a tankless unit.

Solar energy is a clean source of energy and requires less maintenance. This is best suitable to extract energy by erection of solar panels where hot climates in general exist, especially in places like India and countries near to equator zone. This also can be put in moderate temperature zones but has a medium effect The temperatures in any place will be fluctuating and it is nature's behaviour and cannot be constant. Harnessing the solar energy can be done by using solar collectors like flat plate (temperature up to 60o C), parabolic trough collectors (temperature up to 200o C) and solar towers can be used (temperature greater than 200o C). Hence choose of the type of collector depends upon the place and terrain conditions. In response to movement of sun, tilting mechanism is designed so that the collector will be focussed to sun as the sun moves from one end to other end in a day.