Culture of Peace Initiative

District heating seems to be undergoing a renaissance... at least in Europe.  There are a number of projects, especially in Germany and Denmark (as well as the Netherlands and Norway), that are really taking this concept to the next level.  For a good study of the German projects to date, here's a pdf from the International Solar Energy Society's 2005 Solar World Congress.    My opinion?  We should have it here, too.  For a lot of big projects, hub based heating systems just make sense.  But they present some unique challenges.

First off comes the question of storage - Especially in northern climes (like much of Europe), all the energy that's coming in off of summer sunshine is going to dramatically decrease once winter rolls around.  That means that you've got to make the proverbial hay while the sun shines.  And that means you need a place to store all of that collected thermal energy.  Above-ground tanks are an option up to a certain scale and in certain settings.  But once the storage volume required gets significantly large (there are systems in Germany with over 16 Million Gallons of storage!), another alternative becomes necessary - often an Underground Thermal Energy Storage (UTES) system.  A through, well written discussion of these systems, and other alternatives, can be found in this PDF from the Norwegian University of Science and Technology (NTNU).

Storage losses and transportation losses can be huge, as pipe runs spanning from homes, offices, swimming pools and the like all send heat to the surrounding ground and friction losses compound.  Moreover, the tank itself is constantly sending heat out into the ground, especially when it gets significantly hotter than the earth around it.  But in situations where ground water is not constantly moving through the area and carrying heat away, a state of relative equilibrium is soon (in a few years) reached.  Which brings us to the next issue: Solar storage works like a battery.

The thing about batteries is, they need to be charged.  On the scale of heat storage systems in the millions of gallons, the time required to "charge" the storage battery is measured in years, with temperature levels rising and falling seasonally based on weather and usage, but generall rising year to year as the surounding soil, water, rock and the storage medium itself all absob heat and begin increasing the "battery"'s overall levels of heat storage.  This is bad at first, as it means that there are less in the way of immediate results for all that trouble.  But once things get going, this is a really nice thing, as it cushions the blow of, say, a really cold winter or cloudy, rainy summer.

So, not all bad.  But there are a couple more things... first of all, digging a big hole (or drilling a lot of little ones, as they did in a much-cited project in Neckarsulum, Germany), is expensive.  Quite expensive if one is strapped for space and needs to put things like parking lots right on top of your storage tanks, like the designers of this site in Eggenstein Germany, the overview of which also has a nice discussion of the tradeoffs between different storage types.

Also at hand is the issue of complexity in engineering arising from distribution of the energy itself - all those pipes going to and from the storage resivoir can be a real engineering challenge.  Then, you've got to figure out how you're getting you hot water and return lines to and from your collector arrays, and where those collectors are all going to be mounted.   And you have to have backup heat just in case something goes wrong with the collectors or the storage tank isn't hot enough.

Something like this really is a large scale engineering challenge.  But district heating is a problem that's been approached many times in the past - the only truly new twist is that the energy is coming directly from the sun (or from waste heat in industrial operations or summer AC), and being stored for later use.  70% of Stockholm is already being heated by district heating - if they can do it in Scandanavia, we can do it here.