Quote:
Originally Posted by Indyplumber
I have a question about storing the extra heat generated during the day. What is the best commonly available medium to use? I have a basement and crawl space to put something in but nothing large. Would a 4ftx4ftx4ft concrete block, very well insulated and buried in the ground, with PEX tubing running through it work? Can't really have a
1-2k gallon tank anywhere. I understand the efficiencies probably wont be good but I want to make a system as cheaply as possible using common materials.
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The best place to start is the heating system itself. The optimum is a large mass, like a thick stone or concrete floor with built in liquid (hydrating) tubing for heating. It takes ages to heat it up, but also ages to cool down. So you can heat it when you have the sun, and let it cool down at night. The more the mass, the better!
A small area, high temp radiator based system, as well as air based systems, are less efficient if you have a variable heat supply like you have from solar power. Turn your aircon/HVAC off, and your house will heat/cool quite quickly.
A second advantage is that underfloor heating (large mass and area) requires a lower temperature. This is not as important when burning fossils, but essential in solar heat and heatpump installations. The more your heating system can cool the solar heat panels, the more energy you can extract.
Also, if you store heat other places than the floor (for instance in the clay/earth/sand under your house), a large mass - low temperature heat storage loses less energy than a small mass - high temperature storage.
In the project I am devising, I am using several ways of storing heat. Long term storage is under the house (which is clay down to about 7 feet). It is being insulated on the sides, which means most of the energy loss is actually up through the floor and into the house - which is no loss at all. Storing heat during daytime, I have a total capacity of just over 1000 USG (including the main heat buffer).
During summer, when production is high and demand is low, I can absorb as much heat during day as possible, into the clay, as well as the 1000 USG water capacity. During nighttime, the water tanks can keep transferring heat into the clay in anticipation of the next day - to prepare them to be able to absorb more heat. I intend to sync weather prediction data into the system, so if the following day is overcast/cold/windy, I will keep the heat in the water tanks for use during that day and possible the next ones.
I am sorry that I cannot give many more details - or proof - yet, as the system has not yet been finished. When it is up and running, I will supply both proof as well as more detailed information of how it is working. Including some graphical presentations which hopefully will be easier to understand than my mumbling above!
Water and PCM are amongst the better heat storage mediums normally. But for large storage capacities, they become expensive and it is not so easy to build a house on top of a 25000 USG tank of water. Although clay, stone and sand generally only hold about half the heat of water, they are usually there, under your house, already. All you need to do is shoot pipes into it, in a way where you can store and possibly extract heat.
As for PCM specifically, the high grade professional stable material is so expensive that it is only worth while if it changes phase on a daily or at least weekly basis. 3/4 of my 1000+ USG water storage capacity is prepared for insertion of PCM modules, should I later choose to do so. They would most likely be set around 120-140F or so - this means I can store more heat on a good solar day compare to a plain water tank. But I need to see first how many days a year the water tanks reach max capacity. I might even consider a mix, maybe a set of ~ 100F and some at ~ 140F to get different "steps" of capacity increase for different situations.
See also "annual geo solar heating" and similar, for more information on this matter.