In the previous post, we looked at the design and some performance elements associated with the solar hot water system and the gas/tankless assist parts as well.
In cloudy cold times it is clear that the system operates exclusively by depending on the gas assist from the tankless water heaters. Cold water from the town of Tiburon enters the home through the solar hot water holding tank but we are then unable, on cloudy days, to use solar energy to raise the temperature of the water in this tank. So then the tankless heaters pull from this storage tank water that is roughly the same temperature as supplied underground from the city.
On sunny days, the solar panels on the roof circulate a fluid that is both water and glycol (about 50/50) and deliver energy in the form of heat down to a heat exchanger and controller near the storage tank. Water from the storage tank is circulated and raised in temperature in the heat exchanger from the water/glycol fluid that is hotter. Over the course of 5-6 hours the storage tank can rise from 60F to nearly 120F on a very sunny day. Finally, when we pull water for showers on a sunny day, the warm water from the storage tank is then heated a bit more by the gas tankless heater. Note that it is much easier for the gas to raise water temperature by 10-20F than it is by 60-70F. All this is as it is supposed to be.
But the fastest rise in temperature occurs from when the tank is cold (60F) to approximately 100F. Above 100F, the system will still increase the heat content of the storage tank but it happens more slowly. Clearly inefficiencies set in above 100F. Greatest of these is that the storage tank itself. As water in the storage tank gets hotter it also loses heat all around the tank to the air. When the tank is above 115F it loses about 10F in 1-2 hours. When the tank is 95F it loses 10F over probably 12 hours.
So it is best for us to take showers and use hot water when the tank is hot and when there is still more heat to be supplied from the sun so that we can store it over night for any early morning showers as well.
And the question asked by this essay is how much are we able or willing to change habits to accomodate the free energy from the sun? Many people are accustomed to having a shower to wake themselves up and as the first preparation for their work day. Clearly this is not the best time for that shower!
In many homes showers are also taken in the evening after a workout, after coming home from school (sports practice, for example) or just to relax before bed and these showers can have most of their required energy supplied by the solar hot water system. But then the storage tank will be fully depleted of energy and of little use for any morning showers since no sun power will be available until the sun is well risen in the morning.
How many of these catch 22’s will we find as we move to lifestyles where more of our energy is supplied by renewable energy sources? How much change in our habits will we make to milk the natural efficiencies in these systems to their fullest?
Tell me what you think.