I was search­ing for a good heat reser­voir. Unfor­tu­nately it seems that all on the marked are far from state of the art (they are prob­a­bly in their class, but see below).

Most of the devices use water to store the energy. I found one (in Europe/Germany) which is using phase change tech­nol­ogy instead of water to store more heat in the same stor­age place (but you need to ask how much it costs and how long they need to deliver, which prob­a­bly means that it is a lot more expen­sive (partly due to lim­ited amount of pro­duc­tion quan­tity) than water based heat reser­voirs). I have read a lot about phase change mate­ri­als (PCM), and it seems there are dif­fer­ent kinds of sil­ica or wax (or other mate­ri­als) which are bet­ter suited to store heat energy, but the only mass-market tech­nol­ogy seems to be water based ones.

This looks strange to me. When I look at his­tory, other mate­ri­als than water where already used a lot in the past (e.g. stones where heated and then they were used in a press­ing iron or as some­thing which is replaced now by a hot-water bot­tle or an elec­tri­cal heat­ing cush­ion in the bed; yes, all this does not involve a change in the phys­i­cal state of the mate­r­ial, but the point is that other mate­ri­als than water where already used in the past), so I do not under­stand what is pre­vent­ing to let PCM based heat reser­voirs going mainstream.

Except for choos­ing the right PCM and obtain­ing it, it does not look hard to build such a heat reser­voir. You can add a heat-exchanger in the bot­tom and feed solar-power there for long-term heat­ing the PCM, another heat-exchanger at the top to heat the use-water and/or heating-water from the heat stored in the PCM, and a 3^rd heat-exchanger (placed at the top too) which you con­nect to your central-heating if you need a lit­tle bit of quick short-term heat­ing of the PCM. I do not know if you need to add some  heat-layers (e.g. by putting a big cheat of a non-PCM mate­r­ial between the long-term heat­ing part and the short-term heat­ing part), but  it should be easy to test if some­thing like this is ben­e­fi­cial or not. If you have a fire­place which you want to con­nect to the long-term heat­ing of the PCM, it may also be ben­e­fi­cial to have a 4^th heat-exchanger together with the solar-one, but maybe there is another solu­tion to do this with the 3-heat-exchangers-setup (I have not inves­ti­gated this pos­si­bil­ity at all).

If some­one knows some inter­est­ing prod­ucts in Europe or has some help­ful infor­ma­tion (any­thing which can be inte­grated into exist­ing heat­ing sys­tems with­out much ren­o­va­tion of a lot of rooms), please write a comment.

I added two car related items on my ideas page:

• Constant-power cruise control
• Per­sonal GPS tracking

A while ago I got a mail from my bank. They offer to fund a pho­to­voltaic sys­tem on my house with a spe­cial offer. They give a credit upto a spe­cific amount if I use it to install a pho­to­voltaic sys­tem, and to pay it back I just have to give them the com­plete amount of money which I get from the power com­pany when the pho­to­voltaic sys­tem is pro­duc­ing power (the state requires the power com­pany to pay a spe­cific amount of money — fixed dur­ing 20 years — for each kW/h). They also offer that a spe­cial­ist vis­its me to explain the pho­to­voltaic stuff and cal­cu­late if all the phys­i­cal con­straints for such an instal­la­tion are OK.

At first this sounds nice, but I do not trust the words of some ran­dom per­son which wants to get money from me. So I had a deeper look at it.

The roof of my house is still OK, but in some years (maybe 3–5) it has to be changed (at least partly). The roof  is already 30 years old, so this is OK. If I would install a pho­to­voltaic sys­tem now, it would have to be removed and rein­stalled when the roof is changed. If the sys­tem is financed by the bank, this has to be done by an offi­cial spe­cial­ist (instead of let­ting a good friend with expe­ri­ence doing it in exchange of my work­force for some ren­o­va­tion project at his house). This means it would be cheaper if I change the roof before the pho­to­voltaic sys­tem is installed. I do not have the money to do this com­pletely out of my pocket, so the bank has to finance this.

Let­ting the cost of the roof aside (which has to be done “soon” any­way), the inter­est­ing part now is what do I get when the bank is com­pletely financ­ing the pho­to­voltaic sys­tem (maybe parts of the elec­tri­cal instal­la­tion need to get updated, as they are 30 years old too). I have to take into account taxes, what needs to be paid back to the bank, what I have to pay for the power, and what I have when every­thing is paid back.

If the pho­to­voltaic sys­tem is financed com­pletely by the bank, the typ­i­cal cal­cu­la­tion is that it takes about 20 years to pay back every­thing. This assumes I only give to the bank the amount of money I get from the power com­pany for the power of the pho­to­voltaic sys­tem. So basi­cally the pho­to­voltaic sys­tem pays itself. This sounds great, the prob­lem is that a rough esti­ma­tion of the life­time of a pho­to­voltaic sys­tem is 20 years (and after 10 years you may have to change the AC con­verter). The war­ranty on pho­to­voltaic ele­ments here is 2 years, so far I have not seen any offer where they extend it much (some­how it is hard to find some good pages for pri­vate cus­tomers, most of the pages I see are either light on info, or tar­get com­mer­cial cus­tomers). This is not even remotely in the range of 20 years. So after it paid back itself to the bank, it may be bro­ken. And dur­ing those 20 years, I still have to pay my com­plete power bill.

There is also a tax point of view to take into account. Luck­ily a friend of my sis­ter is doing some tax con­sult­ing for pri­vate tax issues. The tax stuff involved here is about com­pany taxes (you are required to open a com­pany when you install a pho­to­voltaic sys­tem here). This is not spe­cially in the area where she is work­ing in, but I assumed she should know enough about the basics, that I can get at least an overview. I got even more than that. When I talked to her about it, she told me they cal­cu­lated a sim­i­lar offer for their house recently. She assumed a pho­to­voltaic instal­la­tion of 20 T€ and that the bank is financ­ing it com­pletely. The roof of their house does not have the same char­ac­ter­is­tics than my house, but as the pho­to­voltaic spe­cial­ists cal­cu­late with rough peak sun­shine hours any­way, it gives me a rough overview. The good part is, that you do not lose money this way, but you have to pay the tax directly (even if I give all the money from the power com­pany to the bank), you start to earn money near the end of the 20 years.

Accord­ing to her cal­cu­la­tions you will have earned 6 T€ after 20 years, when the pho­to­voltaic sys­tem paid itself back to the bank and you had to pay the taxes each month/year/whatever. This assumes that the peak sun­shine hours are not get­ting worse, and that the effi­ciency of the pho­to­voltaic sys­tem stays at roughly the same level dur­ing the 20 years. Now let us cal­cu­late the amount of money per month and per year you gain when you do this:

€/year

€/month

So for 25 €/month I have to invest my time into tak­ing care about the tax stuff, have to take care about the pho­to­voltaic sys­tem (clean­ing, and man­age­ment stuff in case some­thing breaks because of bad weather or nor­mal usage), have to pay extra money when some­thing has to be done to the roof, and so on. And when every­thing is fin­ished after 20 years, the pho­to­voltaic sys­tem may be fin­ished too (and the rate of money the power com­pany has to pay for each kW/h then is not known, in case the sys­tem still works good then). If it is in a state where I have to remove or replace it, I also have to pay the cost of remov­ing it. And I need to get together enough money to get a new roof now.

Now this offer does not sound so good any­more (remem­ber, I also have to pay the power I con­sume dur­ing this time). To me it looks like only the bank and the com­pany installing the sys­tem will ben­e­fit from it, and I have to take care about all the unpleas­ant things.

There is for sure an eco­log­i­cal aspect to think about here. It sounds great to pro­duce green energy (I do not know how much CO2 is pro­duced dur­ing the pro­duc­tion of the pho­to­voltaic sys­tem, and if it will pro­duce enough energy to com­pen­sate this), and I would do it directly if I would see a sane return of invest­ment, but this offer does not look sane in my cur­rent situation.

It seems I have to wait until the prices go down more (and I have some spare money to invest with­out the need for a bank). Recently I have read that the expec­ta­tion is that in about 3–5 years the tech­nol­ogy to print pho­to­voltaic cells can be pro­duc­tion ready, which could reduce the price to a tenth of what a pho­to­voltaic cell costs ATM. Invest­ing 20–30 T€ is not some­thing I would do from one day to another, 4–5 T€ for a sim­i­lar instal­la­tion sounds more easy to agree to.

While I am at it: so far I only see pho­to­voltaic info regard­ing the peak power a cell can gen­er­ate. To me this does not look inter­est­ing. The peak power will be gen­er­ated most of the time in sum­mer (direct sun­light, no clouds, long sun­light times), but in the sum­mer the con­sump­tion of power is less than in win­ter (less light to use, more time spend out­side the house so less time in front of a TV, more BBQ so less cook­ing in the kitchen, …). More inter­est­ing to me would be a good power gen­er­a­tion in less than opti­mal sit­u­a­tions like in win­ter when it is cloudy (but it is prob­a­bly hard to come up with an use­ful value, as the aver­age for “less than opti­mal” varies from loca­tion to loca­tion). I have seen a report about water heat­ing (attached to the cen­tral heat­ing) with solar energy, where a sys­tem is used which has a lower peak power, but a bet­ter power out­put in less than opti­mal weather con­di­tions (round col­lec­tors where used instead of flat pan­els). So the­o­ret­i­cally there is the pos­si­bil­ity to get more power out in not so good con­di­tions, but so far I did not find any infor­ma­tion if it is tech­ni­cally pos­si­ble ATM to opti­mize pho­to­voltaic cells to such sit­u­a­tions, and if it is if there are some cells avail­able for a sane price.