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Piwoslaw · 11-25-10, 03:04 AM

I found a thread on heat buffers (heat batteries, accumulators) on a Polish builders' forum. The thread's author, Adam_mk, seems to know quite a lot, and since this is really interesting, I've translated its essence for others to read. I've only condensed the first post, with some information from later posts, but if someone wants to go through all 160-something pages (with 60 posts per page

), then they can try it through google translate. Thanks to Adam_mk for starting the thread, and thanks to many others for adding information and pictures of how they built their own buffers.

The philosophy behind a heat buffer or heat battery is to "charge" it with thermal energy whenever it is cheaply available, and use it when it is needed. The more energy that can be stored for a longer period of time, the better, so the buffer has to be large (1000-2000 liters of water, 1.5-2.5 tones of thermal mass) and very well insulated (5cm of mineral wool with a layer of aluminum foil, then 30-40cm of styrofoam).

Most commercially available hot water tanks don't have a very efficient design, the internal heat exchanger is coiled around the perimeter. The lowest coils can still give off their heat, but the upper coils sit in the heat released by the lower coils, which reduces heat exchanging dynamics. That cloud of water heated by the coils rises along the walls to the top, while cooler water falls down the center of the tank. Between these two streams of water is an area of turbulence, causing part of each stream to mix with the other. This leads to the hot water not being as hot as it could be, while the cooler areas inside the buffer are warmer than they should be.

The best remedy for this is a buffer that keeps water at different temperatures in different layers, without mixing. Consider the following design:

The coils from a heat pump or solar collector are at the bottom of the tank, and spiral down and outwards. This places them in the coolest region of the buffer, so more heat can be extracted. The water which recieves that heat can rise without washing over more coils, so those are also sitting in cooler water. The rising warm water is collected by a funnel and goes up the middle of the buffer inside of a "chimney", which delivers the hottest water straight to the top, pushing cooler water down, but with no mixing along the way. The chimney is perforated from about halfway up, with the holes becoming gradually larger towards the top. This allows warm, but not hot, water to exit the chimney at the height (level) where water of that temperature is collecting, pushing cooler levels down, but not interfering with the warmer levels above.

Water for domestic use (sink, shower, laundry) is warmed in a coil on the perimeter of the tank. Entering at the bottom, the coils move closer together as they work their way up. Notice that this design qualifies as a double-wall heat exchanger between potable water and the freon or glicol filled coils from the solar/heat pump system.

When heat pump or solar heat is not available, the buffer can be heated ("charged") with a furnace, stove or fireplace - cool water is taken from the bottom and hot water returned at the top. This allows heat from a fireplace/stove to be saved for later, and allows the furnace to always run at its maximum efficiency, instead of having to turn on often for short runs at a lower temperature.

Somewhere at the bottom of the tank there should be 2-3 electric heating elements (1500 watts each) to charge the tank when other heat sources are not available and when electricity at night is cheap.

Using hydronic floor heatering, with its large surface area and low temperatures, will allow much more heat to be used from the buffer, as opposed to normal radiators which require higher temperatures.

One more secret:

All of the inputs and outputs are curved in such a way that water that is leaving/returning to the tank causes the whole mass of water inside to delicately turn in one direction. This buffers the entering/exiting water's kinetic energy without causing turbulence and mixing between thermal levels.

For this design to work well, it should be taller (at least 6ft/2m) than wider (27-35in/70-90cm), so that each thermal level has room to store energy, while the area between levels is minimal.

A heat battery with 1-2 tons of water is a good start, but can later be expanded if required. Here is a link to the Polish thread on how to connect two or more water tanks for maximum efficiency. Of course, using one large tank would be even more efficient (less total surface area = less heat loss), but space or financial restrictions may not allow that.

If you are trying to store the most heat in the smallest possible space, then you can look at using phase change materials to increase capacity. Paraffin wax is a by-product of the oil refining process and many refinaries have more of it than they can get rid of. This material is very good, since its melting point of around 55-60°C is the temperature that you'll usually have in your heat buffer, and its volume doesn't change enough to be much of a problem. The paraffin tank should have lots of small diameter tubes (more tubes = more surface area = better heat exchange) for water to deliver and recieve heat. Here is a link to the Polish thread.

Heat buffers of the DIY variety are the best: they are individually tailored to each house's energy needs and space limitations, and they are much cheaper than a commercial alternative. People who have made one (info in the thread linked at the beginning of the post) said it costs around 5000-6000PLN (around US$2000), while a commercial tank costs almost twice as much and is less efficient. A homemade heat battery is usually adapted from a large commercial tank, or from a pipe segment with custom endcaps welded (bolted) on. The metal can be normal steel, around 4mm thick. Commercial units are often made from stainless steel, but this only raises the cost and makes it harder to weld, while the "stainlessness" is never needed: you won't have a nice shiny cylinder to look at since it'll be under at least 35-45cm of insulation, while on the inside the surface will get covered with a layer of scale after the first time it's heated to 80°C, after that the water will contain no gasses or minerals, so there shouldn't be any corrosion.

Check out AC_Hacker's post for some great links to similar buffers.

11-25-10, 03:04 AM	#3
Piwoslaw Super Moderator Join Date: May 2009 Location: Warsaw, Poland Posts: 961 Thanks: 188 Thanked 110 Times in 86 Posts	I found a thread on heat buffers (heat batteries, accumulators) on a Polish builders' forum. The thread's author, Adam_mk, seems to know quite a lot, and since this is really interesting, I've translated its essence for others to read. I've only condensed the first post, with some information from later posts, but if someone wants to go through all 160-something pages (with 60 posts per page ), then they can try it through google translate. Thanks to Adam_mk for starting the thread, and thanks to many others for adding information and pictures of how they built their own buffers. The philosophy behind a heat buffer or heat battery is to "charge" it with thermal energy whenever it is cheaply available, and use it when it is needed. The more energy that can be stored for a longer period of time, the better, so the buffer has to be large (1000-2000 liters of water, 1.5-2.5 tones of thermal mass) and very well insulated (5cm of mineral wool with a layer of aluminum foil, then 30-40cm of styrofoam). Most commercially available hot water tanks don't have a very efficient design, the internal heat exchanger is coiled around the perimeter. The lowest coils can still give off their heat, but the upper coils sit in the heat released by the lower coils, which reduces heat exchanging dynamics. That cloud of water heated by the coils rises along the walls to the top, while cooler water falls down the center of the tank. Between these two streams of water is an area of turbulence, causing part of each stream to mix with the other. This leads to the hot water not being as hot as it could be, while the cooler areas inside the buffer are warmer than they should be. The best remedy for this is a buffer that keeps water at different temperatures in different layers, without mixing. Consider the following design: The coils from a heat pump or solar collector are at the bottom of the tank, and spiral down and outwards. This places them in the coolest region of the buffer, so more heat can be extracted. The water which recieves that heat can rise without washing over more coils, so those are also sitting in cooler water. The rising warm water is collected by a funnel and goes up the middle of the buffer inside of a "chimney", which delivers the hottest water straight to the top, pushing cooler water down, but with no mixing along the way. The chimney is perforated from about halfway up, with the holes becoming gradually larger towards the top. This allows warm, but not hot, water to exit the chimney at the height (level) where water of that temperature is collecting, pushing cooler levels down, but not interfering with the warmer levels above. Water for domestic use (sink, shower, laundry) is warmed in a coil on the perimeter of the tank. Entering at the bottom, the coils move closer together as they work their way up. Notice that this design qualifies as a double-wall heat exchanger between potable water and the freon or glicol filled coils from the solar/heat pump system. When heat pump or solar heat is not available, the buffer can be heated ("charged") with a furnace, stove or fireplace - cool water is taken from the bottom and hot water returned at the top. This allows heat from a fireplace/stove to be saved for later, and allows the furnace to always run at its maximum efficiency, instead of having to turn on often for short runs at a lower temperature. Somewhere at the bottom of the tank there should be 2-3 electric heating elements (1500 watts each) to charge the tank when other heat sources are not available and when electricity at night is cheap. Using hydronic floor heatering, with its large surface area and low temperatures, will allow much more heat to be used from the buffer, as opposed to normal radiators which require higher temperatures. One more secret: All of the inputs and outputs are curved in such a way that water that is leaving/returning to the tank causes the whole mass of water inside to delicately turn in one direction. This buffers the entering/exiting water's kinetic energy without causing turbulence and mixing between thermal levels. For this design to work well, it should be taller (at least 6ft/2m) than wider (27-35in/70-90cm), so that each thermal level has room to store energy, while the area between levels is minimal. A heat battery with 1-2 tons of water is a good start, but can later be expanded if required. Here is a link to the Polish thread on how to connect two or more water tanks for maximum efficiency. Of course, using one large tank would be even more efficient (less total surface area = less heat loss), but space or financial restrictions may not allow that. If you are trying to store the most heat in the smallest possible space, then you can look at using phase change materials to increase capacity. Paraffin wax is a by-product of the oil refining process and many refinaries have more of it than they can get rid of. This material is very good, since its melting point of around 55-60°C is the temperature that you'll usually have in your heat buffer, and its volume doesn't change enough to be much of a problem. The paraffin tank should have lots of small diameter tubes (more tubes = more surface area = better heat exchange) for water to deliver and recieve heat. Here is a link to the Polish thread. Heat buffers of the DIY variety are the best: they are individually tailored to each house's energy needs and space limitations, and they are much cheaper than a commercial alternative. People who have made one (info in the thread linked at the beginning of the post) said it costs around 5000-6000PLN (around US$2000), while a commercial tank costs almost twice as much and is less efficient. A homemade heat battery is usually adapted from a large commercial tank, or from a pipe segment with custom endcaps welded (bolted) on. The metal can be normal steel, around 4mm thick. Commercial units are often made from stainless steel, but this only raises the cost and makes it harder to weld, while the "stainlessness" is never needed: you won't have a nice shiny cylinder to look at since it'll be under at least 35-45cm of insulation, while on the inside the surface will get covered with a layer of scale after the first time it's heated to 80°C, after that the water will contain no gasses or minerals, so there shouldn't be any corrosion. Check out AC_Hacker's post for some great links to similar buffers.