Heat accumulator

[Piwoslaw]

I'd like to share something I recently found out about how my local power plant uses a new method for raising its efficiency, thereby reducing emissions. But first some background.

Most of the power plants in my climate zone are geared towards producing heat, electricity is a by-product. Of the four power plants in Warsaw only two generate electricity, the other two are shut off at the end of each heating season. The heat feeds the city's warm water network, providing heating for most of the high-rise apartment buildings during the winter, and warm water in the faucets year round. This is more efficient than local, distributed heating. One of the downsides is that it's very hard to adjust production when demand changes hour-to-hour. Most of the electricity is needed in the morning and evening, with a dip at mid-day and a minimum at night. On the other hand, heat production is needed most during the night. Some power plants have extra furnaces that get fired up when demand goes up, but this takes around 3 hours and wastes lots of fuel (almost all power plants here are coal fired, but more are building biomass furnaces). When generating heat at night, the power plant can generate lots of electricity at almost no cost, while during the day the situation is reversed.

About 1-2 years ago the largest power plant in Warsaw decided to add a heat accumulator, already in use at many Skandinavian plants. It's basically a huge thermos which gets warmed up with waste heat during the day, relieving the night load. Or it can be heated at night the help generate electricity in the day. This is a VERY simple setup: just a 47m-high cylinder, 30m in diameter, holding over 30.000 cubic meters (30mln liters) of water. No extra machinery or heat exchangers, just an extra valve. Its heat capacity is 1300MW(thermal). The cost was about US$17mln. I can't find much info on it, nothing on how much it actually saves, but I did find out from an insider that the power plant is very happy with it. So much so that other plants are planning on building something similar.

On a smaller scale, I've read that water-based household heating systems sometimes employ an insulated buffer tank to collect heat when it's available (solar) or cheap (night tariff). I belive that the rule of thumb is around twice the volume of water in the pipes and radiators of the house.

On an even smaller scale, the genII Prius has a thermos for storing engine heat.

[Daox]

Good idea. It sounds like it should be pretty cheap to implement (vs the savings it generates). That is a BIG thermos though.

[Piwoslaw]

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.

[Piwoslaw]

Here are pictures of what the hp/solar coils should look like:

It is actually two coils, plumbed in parallel, one on top of the other. The spirals are 15mm copper tubes, the common input/output is 22mm.

Here is the coil fixed to the bottom cap. Notice the heating element inside the coil. Two more elements will be installed (arrows).

The chimney (before perforating the top half):

The heat exchanger for domestic hot water. Notice how the coils get closer to each other near the top.

The inside of a (partailly completed) heat buffer. Notice the how the inputs/outputs will cause the water to rotate. The direction of rotation should be tailored to the Coriolis effect in your part of the globe.

[Piwoslaw]

The heat buffer is a great place to dump excess heat from air conditioning or ventilation, to use later as hot water or heating. If more than one heat source is used (not counting furnace, stove, fireplace, etc.), then more than one set of coils at the bottom of the tank may be needed. The most efficient way to place them is one under the other, but with a second funnel between them, going to a second, smaller chimney. The cooler of the two sources (heat pump) should use the outer chimney, while the hotter (solar) should use the inner chimney to send its heat to a higher level.

Electric heating elements can not charge the buffer using cheaper night tariffs, but can also be a dump load for wind, hydro, PV, etc.

It's good practice to put 4-8 temperature sensors inside the buffer at different levels to know how much energy is stored. Only one sensor is not enough, since heat is stored in different temperature layers, so it's possible to have a thin layer of 80°C at the very top while most of the rest will be around 50°C, or to have 80°C in the top 70% of the tank. In the latter case the buffer has much more stored energy.

[osolemio]

Very interesting!

I have a 1000 liter tank (~300 USG) which is made for this already, from the Austrian Okofen (US dealer is located at oekofen-usa dot com). The one I have has two heating circuits, a solar heat exchanger and hot water is flow-through in a stainless steel pipe, not "sump water" (always fresh water, both hot and cold).

The solar heat exchanger is at the bottom, but it has a "chimney", an internal vertical tube connected, with holes in it. The hot water in the buffer water will exit from this chimney at exactly the level where it does most good, as the density of the water changes slightly with temperature. If there is plenty solar heat, it goes to the top to give a good temperature for the hot water supply, leaving the bottom less heated for times where there is less heat from the solar panels.

[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.

[AC_Hacker]

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.

> What is the best commonly available medium to use?

Best can have many meanings.

You might want to find out what "specific heat" means, it might help you to answer your question.

-AC_Hacker

[Piwoslaw]

I've read about storing heat in phase-change materials.

One idea to keep automotive engine coolant warm was a phase change salt, see this thread over at EcoModder for more info.

Another idea, on a larger scale, is to use wax (paraffin). Basically, you need a well insulated container with lots of small tubes crisscrossing, then you fill that up with the right type of wax. Paraffin is a by-product of oil refining, I read somewhere that many refineries would be glad to get rid of their extra paraffin. Make sure you get the type with melting point around the temperature you want to keep your heat stored. Once everything is working, water flowing through the tank will be warmed up - you'll see it's exit temperature gradually decline, then flatten out for while (melting temperature of the wax), then decline again. The rate at which heat is tranfered from wax to water (or water to wax when charging) depends on how close together the tubes are, how much surface area they have, etc.

Parafin tanks work great when paired with a heat buffer.

I can't remember the exact numbers, but I think that a paraffin tank can store something like 30%-80% more heat than a water tank of the same size. If anyone is interested I can try to find more info.

[osolemio]

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.

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.