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Well this is embarrassing
for me.
It seems I was wrong and I will try to explain how I setup the virtual experiment to explain the problem (to myself).
I consider ambient temperature +27C and have 3 barrels each containing 100liters of water each at different temperature one +27C same as ambient second at +17C and last one at +7C
First peltier say a TEC1-12706 will be in contact with the +7C barrel on the cold side and in contact with the +17C barrel on the hot side.
Since this powered at 3.69W and with a COP of 3.25 according to datasheet will extract 12W it means that the barrel will cool by 0.1C in one hour of operation (this was the reason I selected 100 liters so temperature will not change much so COP will be relatively constant).
The barrel on the hot side will heat up slightly more by 0.13C in the same hour since all of the energy from first barrel is transferred there 12Wh + the energy used by the peltier 3.69Wh
Now the third barrel that is at +27C is thermally connected with the hot side of the second peltier element and the cold side will be of course connected to the second barrel that is at +17C.
Considering the first +7C barrel thermally isolated so that it will lose 12W to ambient then second barrel ideal insulated no loss and third barrel not insulated at all so that it will mostly stay at +27C that happens to also be the ambient temperature.
Now is quite clear that while this will be running the first barrel will stay at +7C because it will be cooled at a rate of 12W while the loss to ambient will heat it back at a rate of 12W so temperature will remain constant.
The second barrel will also remain constant since the second peltier will extract the necessary 15.69W from the cold side while the 3 barrel will receive the 15.69W + the 4.8W so total 20.49W that will be transferred to ambient since the 3 barrel is not thermally insulated at all.
Now the COP for cooling will be just 12W / (3.69 + 4.8 ) = 1.41 while for heating COP will be 2.41
I think the best way is to add a remark in the original post so people ignore all the long discussion before.
Thanks to all involved for clarifying this for me. It seems I was the one that had a hard time understanding this.
Since this is now closed (I think unless I'm wrong again) you can ask question about my system DMPPT that has nothing to do with heat pumps. -
While same module used at just 3V will require 1.23A at a delta of 10C so will move 12W wile requiring just 3.69W so a COP of 3.25
Right. That first module will move 12W from the cold side with delta T of 10C and running at 1.23A:QuoteIf I where to stack another module I will probably be using the TEC1-12707 with the cold side in contact with the hot side of the TEC1-12707 thus at the same 3V I will be able to move the necessary 12W + 3.69W = 15.69W at 10C delta while consuming about 4.8W 3Vx1.6A and maintaining the same COP 3.25
Yes, fine so far...QuoteThis two TEC seen as a single unit will be capable of moving a total of 27.69W from the cold side
But that is where you keep going wrong. The total moved from the cold side (of the first module) is still 12W - after all it's still being driven at 1.23A and still has a delta T of 10C. It doesn't know that you just replaced its hot side heatsink with another peltier - how could it tell? There's nothing in the data sheet that says it will pump 12W @ 1.23A, 10C delta T *unless the hot side happens to be connected to another peltier instead of a heatsink*
If you want it start pumping 27.69 W from its cold side then you will have to more than double the current from the original 1.23A. And the 2nd module will also require a lot more power to move 27.69W instead of 15.69W. Anybody hear the sound of rapidly deflating COPs?
You have constantly ignored my statements that the second peltier is only moving the 12W provided by the first stage and not adding *another* 12W of heat pumping - it could only do that if they were in parallel, but then the total delta T would only be 10C. If the penny still hasn't dropped then please don't present yet another scenario without addressing the fundamental question as to why the heat pumped by the first stage would change due to the presence of another peltier on its hot side.
I gave you a link to a peltier manufacturer's technical data showing the equations for two and three stage coolers. They are quite straightforward being mostly constants apart from the driving currents and the temperature differences. Please look at them where you can clearly see that the heat pumped from the first stage is the same in the room and three stage cases. (Note that they number the stages starting from the hottest stage).
Finally, there is loads of information available on the net. All of which says that cascading peltier is useful if you want delta Ts higher than around 70C and that peltier are very inefficient for delta Ts above approx 20C. Do you really think that it never ocurred to any of the experts, including the manufacturers, to simply run them somewhat below their full power and reap such rewards? After all, as you say they are dirt cheap compared to small compressors even if you have to use only 25% of their max capacity. -
Well this is embarrassing
for me.
It seems I was wrong and I will try to explain how I setup the virtual experiment to explain the problem (to myself).
Ah okay. Oddly the forum didn't warn me that your last response had been posted whilst mine was being prepared - it usually does.
Glad you've finally got it.
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Ah okay. Oddly the forum didn't warn me that your last response had been posted whilst mine was being prepared - it usually does.
Glad you've finally got it.
Thanks for insisting I was quite sure I was right
Luckily is cold enough here that I do not need cooling in summer so I was not thinking much about the problem.
I have a small fridge that was designed to be used in a car just 40 liter volume and uses one of those peltier probably the TEC1-12706.
There is nothing else other than the peltier element two heat sinks (one on each side) and two fans.
It was a bad idea since it will freeze everything inside with normal 20 to 28C ambient temperature while using about 50W
I reduced the voltage to 5V in winter and there it takes about 2A 10W including the fans that are inaudible and it manages to create a 17C delta between ambient and inside that is just right.
In summer when is a bit warmer inside I increase that voltage to about 6V and it uses about 13W (this also includes the DC-DC converter from 26V battery) and then delta is around 20C
In the past when I got a few peltier modules at 2 or $3 it was to experiment with them as generators and there they where about 3 to 4% efficient based on rough measurements.
Back then I was thinking on using thermal solar to heat a large thermal mass and then generate electricity as maybe a better combination than PV plus electrochemical battery.
Since then the PV panes drop substantially in price and the entire idea made no more economic sense.
Of course thermal mass storage is and will probably ever be one of the most economical ways to store energy. -
Storing thermal energy is usually not that cheap, at least small scale. It can get efficient when used large scale, as the surface only scales like square while the mass scales with the third power. You not only need the thermal mass and insulation, but also heat exchangers to get the energy in and out - these can be problem when using a solid medium like rock. large containers also get difficult to transport - especially to remote areas. It also needs quite some space - so you might need extra protective structure.
One problem with storing heat for the winter is that this gives you not much more than one charge / discharge cycle per year. So it may be only 20-50 cycles over the life. Things get a little better for shorter time storage like day - night or maybe weekly for hot water. But still not that many cycles.
There are a few tries to store solar thermal energy for the winter - these usually need to be integrated when constructing the house and they are not really cheap, with storage being a large part of the costs, not the solar thermal collectors. -
Earth sinked heat pumps are the only successful application of long term heat storage that I know about.
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Storing thermal energy is usually not that cheap, at least small scale. It can get efficient when used large scale, as the surface only scales like square while the mass scales with the third power. You not only need the thermal mass and insulation, but also heat exchangers to get the energy in and out - these can be problem when using a solid medium like rock. large containers also get difficult to transport - especially to remote areas. It also needs quite some space - so you might need extra protective structure.
One problem with storing heat for the winter is that this gives you not much more than one charge / discharge cycle per year. So it may be only 20-50 cycles over the life. Things get a little better for shorter time storage like day - night or maybe weekly for hot water. But still not that many cycles.
There are a few tries to store solar thermal energy for the winter - these usually need to be integrated when constructing the house and they are not really cheap, with storage being a large part of the costs, not the solar thermal collectors.
I was not talking about long therm thermal storage just a few days to cover for solar energy availability. You can see a few example of thermal storage in my presentation.
In my particular case the house was designed with build in thermal storage at almost no additional cost since the thermal storage is also the structural part that was needed anyway and is comprise of a 14 cubic meter concrete floor/foundation.
Since is both thermal storage and radiator there is no need for any extra insulation since is inside the insulated house that needs heating and also no heat exchangers needed all is solid state.
Since the max temperature delta is limited at about 12C the storage capacity is 97.4kWh. The house heating needs are 1000kWh for the coldest month January here so a capacity of 97.4kWh should be good for max 3 days with zero energy from the sun. Of course zero energy from the sun is not realistic since even in the worst overcast days there is still significant amount received.
So yes I agree with you in regards to seasonal thermal storage and that is expensive but short therm solar energy storage to average the solar output so overnight and a few cloudy days is extremely cost effective.
Another example I used was an inexpensive plastic barrel with 206 liter capacity filed with water and an allowed temperature delta of 35C that will give a total storage capacity of 8.3kWh. If this is inside the space that needs to be heated and the thermal loss to ambient is calculated so that almost no thermal insulation is needed then again no heat exchangers will be needed. Heating will be done the same way as the concrete floor with heating wires (basically just normal copper wires).
The cost of the barrel is 60USD new and water is basically free the heating wires needed are also inexpensive and cycle life is not an issue so you can imagine the comparison with a similar capacity 8.3kWh Lithium battery that has also a limited life and will probably need at least one replacement over a 25 to 30 year amortization period.Earth sinked heat pumps are the only successful application of long term heat storage that I know about.
See what I mentioned above and geothermal heat pumps are not cost effective for sure not cost effective at my location where just the geothermal loop will cost more than my entire solar PV heating instillation. -
Thermal storage in the concrete structure will usually not allow for a 12 K temperature window - this would be rather uncomfortable in most cases. It might be OK with special insulation at the walls, but not with normal construction. Just water barrels in the basement also need the room - so you have so take into account those costs. Usually one will also need some regulation, like fans of similar - other wise it is either way to warm or not giving much heat to the house. Without control of the heat flow, the useful temperature window would be more like 3-5 K, like a normal concrete structure.
It depends on the local climate - but even in really good areas there can be something like a week with snow cover or a few days without much sun. Similar really cold days / weeks might need twice the normal heat - so one needs a buffer for these days too. The combination of snow fall followed by a week of cold temperatures is not so rare. Worst case one can no rely on the snow to slide down by itself. So storage for 1 (or more) week at twice the normal power would be a good idea, if there is no backup solution. You won't need it every year, but one might.
At least here, on a overcast or foggy day, solar power is down to the 1% range - so hardy enough to start the PV. -
Thermal storage in the concrete structure will usually not allow for a 12 K temperature window - this would be rather uncomfortable in most cases. It might be OK with special insulation at the walls, but not with normal construction. Just water barrels in the basement also need the room - so you have so take into account those costs. Usually one will also need some regulation, like fans of similar - other wise it is either way to warm or not giving much heat to the house. Without control of the heat flow, the useful temperature window would be more like 3-5 K, like a normal concrete structure.
It depends on the local climate - but even in really good areas there can be something like a week with snow cover or a few days without much sun. Similar really cold days / weeks might need twice the normal heat - so one needs a buffer for these days too. The combination of snow fall followed by a week of cold temperatures is not so rare. Worst case one can no rely on the snow to slide down by itself. So storage for 1 (or more) week at twice the normal power would be a good idea, if there is no backup solution. You won't need it every year, but one might.
At least here, on a overcast or foggy day, solar power is down to the 1% range - so hardy enough to start the PV.
The example in the pdf presentation if for my particular case.
The house was designed and build by me and is in a fairly cold but sunny climate. Average temperature for the month of January (coldest month here) are -16.5C
This is the forth winter I spend in this house and currently for heating I use small propane tanks that are realy expensive and inconvenient.
Each propane tank contains 8kg of propane that has about 100kWh of stored energy and I use a small and inefficient propane heater that I run in average a few hours a day and that is outside and heats a glycol mixture in a closed circuit loop that then will transfer that trough a heat exchanger energy to an open loop made of a 206 liter barrel of water as a buffer and then circulated trough PEX tube trough my concrete floor.
The most propane tanks I used in a month was 11.5 two years ago when there was a colder winter and I'm at about 6.5 propane tanks so far this month.
If I'm sort of generous and say that the propane burner is 80% efficient that means I need 11.5 x 100kWh x 0.8 = 920kWh for the worst winter month in the past 4 years.
There are days when I can not heat at all due to high wind speeds and the fact that propane heater is outside and while it has now forced air for combustion is just a bad DIY job so it will still not work when wind speed is above 50 to 60km/h and that is fairly common here especially in winter.
The only thermal storage that I have know is that 14 cubic meter concrete floor that can go down to even 16C after 2 or 3 days without heating from a starting point of +21C and that is about as high as I can get now due to the fact that propane heater is limited at +70C since is designed for hot water for shower in camping (a realy cheap China made unit with 10kW max burner power).
Whit solar PV and heating cable I can get the concrete floor up to +28C if necessary and again if necessary it can drop down to a less comfortable +16C so that is where the +12C max range is coming from.
Of course this is a simplification since my house has quite a bit more thermal mass from the solid wood wall (unusual construction and if you realy want you can see the short video I made showing how the house is build )
So I know not just theoretical what the energy input for heating is for my house and also know the amount of solar. The snow here will not cover the panels since panels are at a steep 70 degree angle and is so cold that snow is not sticky. I may need to clean the panels maybe two or three times a year usually at the beginning or end of winter since then is when is warmer and snow may stick to the panels.
Of course each climate is different and so thermal mass and size of PV array will be quite dependent on that.
I guess you live in Germany where there is way less sun in winter but on the other hand the temperatures are not nearly as extreme as here so you need less energy to heat the same house. Also the cost of natural gas is probably significantly higher there than it is here so when you consider all this factors PV heating will still probably be the least expensive heating option even there.
I know most did not opened the pdf presentation and while the graph is present there I will also add it here.
Image is small just 175kb so I hope it will not affect those with slow internet connection.
The graph represent the daily energy generated by a 10kW PV array installed at my location and as you can see the worst case is about 8kWh in January and that worst case is just for a day most other semi cloudy days will be way better.
So in 95% of the cases thermal storage temperature delta will not change by more than 3 to 5C as you mentioned and in those worse days I can still use the available energy to heat the air so air stays at a more constant and comfortable temperature. I will have the heat recovery for house forced air ventilation ready by next winter and that should also save some energy compared with now opening windows each day. And the fresh air output from the heat recovery can be heated to desired temperature by a much smaller thermal mass storage for increased comfort.
Sorry for the long replay. -
electrodacus
Why do you use the propane heater outside? I used one of these for 3 winters inside and I am still alive ha ha I just used common sense like no overnight, closed windows most of the time. It sure helped and I lived in a concrete bunker. Sure got tired of hauling 5 gal propane tanks.
https://www.amazon.com/Dura-Heat-Propane-TankTop-Heater/dp/B002LUSHPW/ref=sr_1_15?ie=UTF8&qid=1484868846&sr=8-15&keywords=propane+heater -
Based on the cost of raw materials TEC should be less expensive than compressor based so is maybe just about the production volume.
The raw material cost for solar (panels?) is no more than the cost of drywall (per Tesla CEO), but being the same p and n doped silicon crystal you'll see why it's obviously not really anything to do with the cost of raw material and most definitely energy.I also suspect energy embodiment is the same driver with Li batteries as[1] it takes only about 80g battery grade Li / kWh or about $2 worth and merely 2% the cost assuming $100 / kWh. In fact the highest cost I found current was 800k RMB/mt or only about $10 worth so at most 1/10 the cost of the fabled $100 point.
Thermoelectric tunneling might hold promise:
http://memim.com/thermotunnel-cooling.html
https://arxiv.org/ftp/arxiv/papers/0807/0807.2527.pdf
[1] http://benchmarkminerals.com/elon-musk-our-lithium-ion-batteries-should-be-called-nickel-graphite/
And cobalt is another to consider. Lifepo4 is probably the most economical chemistry and great for stationary. Specific energy is about half of other chemistries, but for autos the lower energy density makes it unfeasible mainly. -
Why do you use the propane heater outside? I used one of these for 3 winters inside and I am still alive ha ha I just used common sense like no overnight, closed windows most of the time. It sure helped and I lived in a concrete bunker. Sure got tired of hauling 5 gal propane tanks.
Is not just about the carbon dioxide there will also be a lot of water vapor while burning propane and that is bad news when is also that cold outside. I do not remember the exact number but you get about 2x the amount of water by wight compared with the propane that you burn. And since I burn about one of those propane tanks each 3 days all that water vapor will end up inside my house.
On top of that it will get way to hot inside the house and most of the heat will be stratified on the upper level of the rooms not to mention also a lot more will escape trough the windows since they will need to be opened all the time during heating.
The concrete floor will remain cold an uncomfortable while air will be to warm during the day and cold during the night when you do not use the burner.[
The raw material cost for solar (panels?) is no more than the cost of drywall (per Tesla CEO), but being the same p and n doped silicon crystal you'll see why it's obviously not really anything to do with the cost of raw material and most definitely energy. I also suspect energy embodiment is the same driver with Li batteries as it takes only about 80g battery grade Li / kWh or about $2 worth and merely 2% the cost assuming $100 / kWh. In fact the highest cost I found current was 800k RMB/mt or only about $10 worth so at most 1/10 the cost of the fabled $100 point.
Thermoelectric tunneling might hold promise:
http://memim.com/thermotunnel-cooling.html
https://arxiv.org/ftp/arxiv/papers/0807/0807.2527.pdf
Yes in may cases the cost of raw materials is realy low compared to the price of the final product .
Here is a recent paper that tries to give an idea about of the cost of PV panels http://www.nrel.gov/docs/fy16osti/65872.pdf
The paper is a bit detailed but fairly good and also when they talk about cost amortization they consider that for a grid tie connection so includes inverters and other equipment.
I think the future of energy will be solar and decentralized maybe some smart micro grid type stuff in densely populated areas.
Battery cost is similar but for low volume consumer prices as of now they are about $220/kWh capacity for LiCoO2 and similar high energy density cells all in the range of 500 to 1000 cycle life and LiFePO4 with an acquisition cost of $350 to $400 and 2000 to 6000 cycles so more suitable for stationary energy storage.
The fact that LiFePO4 is so expensive has to do with the low volume and demand for this since for now portable electronics and EV have a larger share than stationary energy storage.
Since 60 to 85% of a house energy use is in space heating and water heating the thermal storage can play a huge role and I do not think thermal storage will ever go away since nothing can realy compete with that in therms of cost. -
Heat storage in the foundation can lead to quite some extra heat loss - though mainly in phases when the heat buffer is relatively full and thus not that critical. Still a 12 K temperature window will include relatively low comfort temperatures. If would also be difficult to control the heat - so this should be only a part of the storage. Hot water storage also needs space inside or an extra protective structure and insulation - so it is not that cheap. It might be also a good idea to have some really hot water reserve for washing and similar.
Having a backup system (e.g. the propane gas from bottles) for really cold weeks can ease the buffer requirements a lot, even if you use it only very little. It can save you the storage you only need once a year or even less. It is a good idea to have some backup, just in case something brakes and you need the extra heat to survive.
For the rather low outside temperatures, forced ventilation with energy recovery could be a good way to reduce the energy need even further. They are usually needed to get really low heat requirements - at some point it gets more efficient than extra insulation.
With such a small house, it could be tricky to mound 70 m² at a steep angle. This could increase the installation costs. The 70m² might be enough for heating, but it might need mode for other needs.
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Heat storage in the foundation can lead to quite some extra heat loss - though mainly in phases when the heat buffer is relatively full and thus not that critical. Still a 12 K temperature window will include relatively low comfort temperatures. If would also be difficult to control the heat - so this should be only a part of the storage. Hot water storage also needs space inside or an extra protective structure and insulation - so it is not that cheap. It might be also a good idea to have some really hot water reserve for washing and similar.
Not sure you seen my video on how the house was build. The foundation is a thermal insulated slab on grade. There is no thermal bridging to outside. You can also check the pdf version of the house project where you can see how the house is thermally insulated http://greensask.com/
You can see actually in this image below
Since house is so well insulated and there is large amount of thermal storage it can actually provide all the storage needs or worst case 80% of the storage needs.
I will have a much smaller thermal storage to preheat the air from the heat recovery unit.
I do not need large amount of thermal storage for hot water since there will use most of the water during the day and there will be enough energy even in overcast days to heat the water on demand.
The amount of water I use will also be realy low compared to typical usage.Having a backup system (e.g. the propane gas from bottles) for really cold weeks can ease the buffer requirements a lot, even if you use it only very little. It can save you the storage you only need once a year or even less. It is a good idea to have some backup, just in case something brakes and you need the extra heat to survive.
I was considering a backup system since that way I could reduce the PV array size by about 30% but the thing is that the backups system that will be needed to provide just 10% of the heat while saving the 30% on PV array size will make the entire system more expensive and almost double the cost of heating.
The small propane tanks that I use now cost between 25 and $30 to refill and they contain 100kWh (8kg of propane) of energy and that is 25 to 30cent/kWh compared to energy from PV panels that is 2.4cent/kWh so an order of magnitude less.
Then when you replace 10% of the energy with one that costs 10x more the average price will double. And is way better to have a single system even if you need to double the size of the array and have huge excess of energy even in winter.
Natural gas is way less expensive than propane but that requires a connection that will cost way more than all the PV solar installation so it makes absolutely no sense. Price of natural gas also fluctuates a lot (2 to 3x) and you have no control over that cost.For the rather low outside temperatures, forced ventilation with energy recovery could be a good way to reduce the energy need even further. They are usually needed to get really low heat requirements - at some point it gets more efficient than extra insulation.
Yes I expect to have some reduction is the energy needed when I install the heat recovery unit (rough theoretical calculation sees a 15 to 20% reduction compared to now). And yes you can not realy build an energy efficient house without including heat recovery unit.With such a small house, it could be tricky to mound 70 m² at a steep angle. This could increase the installation costs. The 70m² might be enough for heating, but it might need mode for other needs.
I need about 64m² of panels and the mounting is no problem since it will be a ground mount. My house is on a 8 hectare lot so space is no issue.
If I had considered the PV heating when I designed the house I will have included the PV panels in to the house structure but then the house will needed to be something similar to an A frame type building so that panels are at least an a 60 degree angle so I get the most out of it in winter months and also help with the snow.
Here is a photo of my house when we got first snow this year. There are only 3kW of PV installed and I will need 9 to 10kW to provide 100% of the heating to my house
I will only have the panels on 2 row not three like it is not the case with the 9 x 260W panels so it is easier to install.
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Heating the hot water just when you need it, needs a high power. Years ago we had such a system for the summer - it was rated at something like 22 kW. So at best this would only work if the PV is really supplying 5 KW or so. So no more shower in the morning unless there is at least a small (e.g. 50 L) buffer - going a little larger might no be so much more expensive, and this would be high quality (controllable) storage - more replacing battery than just low temperature thermal storage in the building mass.
I know a backup system will add quite some costs, but it also saves quite a lot on the required storage and possibly a little on the PV. It could be the propane system you already have - the relatively high fuel costs are not that important if you only use it for may be 1-3 % on average. Still that could cut down the storage need by something like 3-5 days or maybe 30%-50%. The main part is to cut away those worst case bad weather phases that may happen every 5 years. Extra storage to the existing concrete can get rather expensive. You can't compare it to just PV, but more to PV+low cycle storage and it is the extra buffer that causes the costs. Even thermal storage gets expensive per kWh if used only once a year or less. I don't think the concrete floor is enough storage for those worst cases, even if you accept the room temperature to go down considerably (e.g. < 15 C) and thus get a little more out than normal. -
Heating the hot water just when you need it, needs a high power. Years ago we had such a system for the summer - it was rated at something like 22 kW. So at best this would only work if the PV is really supplying 5 KW or so. So no more shower in the morning unless there is at least a small (e.g. 50 L) buffer - going a little larger might no be so much more expensive, and this would be high quality (controllable) storage - more replacing battery than just low temperature thermal storage in the building mass.
I know a backup system will add quite some costs, but it also saves quite a lot on the required storage and possibly a little on the PV. It could be the propane system you already have - the relatively high fuel costs are not that important if you only use it for may be 1-3 % on average. Still that could cut down the storage need by something like 3-5 days or maybe 30%-50%. The main part is to cut away those worst case bad weather phases that may happen every 5 years. Extra storage to the existing concrete can get rather expensive. You can't compare it to just PV, but more to PV+low cycle storage and it is the extra buffer that causes the costs. Even thermal storage gets expensive per kWh if used only once a year or less. I don't think the concrete floor is enough storage for those worst cases, even if you accept the room temperature to go down considerably (e.g. < 15 C) and thus get a little more out than normal.
Heating hot water on demand needs more power but I use small amounts of water so for me works. I'm also not a morning person so I will be up only after the sun will be up
Low flow shower heads can provide 4 to 6 liters per minute and in order to heat that water from say 20C room temperature to 40C that most people prefer requires a 20C delta.
Rising the temperature of 1 liter of water by 1 degree Celsius requires 1.16Wh of energy
Say you use 4 liter per minute for 10 minutes that is 40 liters so total energy needed is is 40 x 20 x 1.16 = 928Wh
Since that energy needs to be delivered in 10 minutes the heating power will need to be 928Wh x (60/10) = 5.5kW
The flow rate on my taps and shower is even lower than that at 1 to 2 liter/minute so in my particular case even less power is needed. (But is a particular case).
The added cost of the backup system will be much higher than over-sizing the solar system and thermal storage.
I prefer to spend two or three days each 5 years at a less comfortable internal temperature can even be 17C or 16C than have to deal with a backup system.
Thermal storage is extremely inexpensive. You can build a 160kWh thermal storage for around $2000
Just made a long video explaining how the DMPPT works and the example for my house if you get the time to watch this long video and deal with my bad English
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electrodacus - 10% at 10x cost only increases cost by 90%, not 2x : )
$414 for 14kW mppt is surely good, and $240 for ~14kW mppt capacity is good too (trusting it's mppt):
http://www.ebay.com/itm/5-10-15-20-30A-PWM-Solar-Panel-Battery-Regulator-Charge-Controller-12-24V-DC-/171308053532?var=&hash=item27e2c1741c:m:mD5wQY2jD5vaJDldCLuXVzA
I do like your solution with thermal storage controller. This can be implemented as a 'battery' to standard mppt system as a second battery bank (which is 'charged' once desired temp is reached). -
electrodacus - 10% at 10x cost only increases cost by 90%, not 2x : )
Yes you are right that is way better$414 for 14kW mppt is surely good, and $240 for ~14kW mppt capacity is good too (trusting it's mppt):
http://www.ebay.com/itm/5-10-15-20-30A-PWM-Solar-Panel-Battery-Regulator-Charge-Controller-12-24V-DC-/171308053532?var=&hash=item27e2c1741c:m:mD5wQY2jD5vaJDldCLuXVzA
That is a PWM controller where do you see the mention of MPPT ? Ok just looked again and in description at one point they say MPPT solar charge controller of course it is not but they did not mentioned that in title where is clearly stated as a PWM only charger. So they will say it was just a copy and paste mistake in the description. But none will expect a 30A x 24V = 720W DC-DC converter for $12.
Of course there are plenty of fake MPPT lead acid charge controllers available on eBay.
In any case my Digital MPPT (the digital in front is extremely important ) since it is not a DC-DC converter is a completely different thing and nothing similar exists on the market.I do like your solution with thermal storage controller. This can be implemented as a 'battery' to standard mppt system as a second battery bank (which is 'charged' once desired temp is reached).
Not sure I get this last part. The thermal storage will be massive and will need many days of full sun (probably about a week) to get the thermal storage fully charged (it will almost never happen)
So the Lithium battery charging has priority then all the remaining energy is stored in thermal mass.
But since PV array is so massive to cover house heating the Lithium battery will be charged very fast and over 90% of the energy will still got to the much larger capacity thermal mass. -
[quote author=electrodacus link=topic=81192.msg1118752#msg1118752
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Not sure I get this last part. The thermal storage will be massive and will need many days of full sun (probably about a week) to get the thermal storage fully charged (it will almost never happen)
So the Lithium battery charging has priority then all the remaining energy is stored in thermal mass.
But since PV array is so massive to cover house heating the Lithium battery will be charged very fast and over 90% of the energy will still got to the much larger capacity thermal mass.
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To have a system that will also work in winter and without often using a backup system, the PV capacity must be rather large. So even average winter power should be able to fully charge all the buffers rather fast (e.g. a week) - just a few sunny days would should fill the buffers. So the normal case would be battery and thermal buffer nearly full, and only depleting on a few dark days in a row. It is only a full buffer that really helps. Looking at just a few years data may not be enough to makes sure the buffers / PV capacity is sufficient, even for reasonable unfavorable weather.
With the concrete mass as a buffer there could be a problem with a lot of heat moving to the house one a not so cold day. So one might have to use extra ventilation to cool and thus has higher than normal losses on these days. It is not that bad, but is could effect the efficiency of the buffer. So the 12 C temperature spread looks rather high to me, unless there is some control over the heat release. -
To have a system that will also work in winter and without often using a backup system, the PV capacity must be rather large. So even average winter power should be able to fully charge all the buffers rather fast (e.g. a week) - just a few sunny days would should fill the buffers. So the normal case would be battery and thermal buffer nearly full, and only depleting on a few dark days in a row. It is only a full buffer that really helps. Looking at just a few years data may not be enough to makes sure the buffers / PV capacity is sufficient, even for reasonable unfavorable weather.
With the concrete mass as a buffer there could be a problem with a lot of heat moving to the house one a not so cold day. So one might have to use extra ventilation to cool and thus has higher than normal losses on these days. It is not that bad, but is could effect the efficiency of the buffer. So the 12 C temperature spread looks rather high to me, unless there is some control over the heat release.
How fast the buffer will need to be charged to full depends a lot on location. In a location like mine where no more than 3 days without sun are expected the buffer can be much smaller than a location where one week without sun can be expected.
I do have also chances with 5 to 6 cloudy days but that is in November where average temperatures are much higher than in January so a full buffer in that climate will be able to cover a week the same way that the same buffer can cover 2 or 3 days in the much colder January.
As I mentioned before a fully discharged buffer for me is +16C and a fully charged buffer is +28C but the buffer will mostly work in the +19C to +25C or even smaller range most of the time.
The forced air ventilation can take care of keeping the air temperature inside at even more constant temperature.
In those few extreme cases when the buffer gets to say +16C the energy produced will be used to first preheat the air so that air temperature is maintained around +20C.
And on the other situation where buffer (concrete floor) gets above +25C (that will only happen manually if I expect realy bad whether for many consecutive days else it will be limited to +25C) and then the house air will not be preheated so that air temperature will be kept a bit lover.
The combination of floor temperature and house ventilation can keep the house air temperature in a narrow range of +20C to +23C at all times.
When is cloudy in January there is still energy being produced and not insignificant amounts and that energy is then used to charge a much smaller buffer that will control the air temperature.
If I take the concrete example based on that annual energy graph I have a good period where buffers will get full or close to and the starting with day 9 there is deficit of energy.
The house needs about 900kWh in January in an average year so that is about 30kWh/day to maintain the house temperature.
In day 9 say the concrete floor is at +25C and the concrete floor has a storage capacity of around 8kWh per degree Celsius.
So day 10 I get just around 22.5kWh from the sun that is a deficit of (7.5kWh) so say the concrete floor temperature by the end on day 10 drops by 1C so is now just +24C
Day 11 I get about 20.5kWh so concrete floor temperature will drop by about another degree Celsius and so it will be +23C
Day 12 I get around 42kWh of energy so that will cover the average 30kWh needed plus add another 12kW in thermal storage so temperature is now back again to around +24.5C
I can continue but you get the idea.
In November I only need about 400kWh to maintain the house warm since average temperature for that month is higher even if solar energy is worse with more much darker days than in January but then I only need about 14kWh/day to maintain the temperature so the thermal storage can deal with more consecutive bad days.
Another thing is that usually cloudy days are not as extreme cold as sunny days since the layer of clouds act as a blanket and keep the temperatures a bit higher.
In any case things are a bit more complex and all the interactions but I hope you get what I try to say.
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Of course there are plenty of fake MPPT lead acid charge controllers available on eBay.
In any case my Digital MPPT (the digital in front is extremely important ) since it is not a DC-DC converter is a completely different thing and nothing similar exists on the market.
I had reservation it seemed dubious, however some are advertising PWM + MPPT, which is maybe a little more dubious.
Why are other MPPTs based on MCUs not 'digital'? Can you safely assert all other MPPTs only use linear IC's?With the concrete mass as a buffer there could be a problem with a lot of heat moving to the house one a not so cold day. So one might have to use extra ventilation to cool and thus has higher than normal losses on these days. It is not that bad, but is could effect the efficiency of the buffer. So the 12 C temperature spread looks rather high to me, unless there is some control over the heat release.
Good point. I'm sure all have researched trombe wall, here is one report on efficiency:
http://www.nrel.gov/docs/fy04osti/36277.pdf
That claims nominal 13% efficiency for simple non-insulated design.
What about a fully insulated trombe wall 'structure'. Three sides and bottom have insulation, and the south has double-pane glass with an insulated rolling curtain to cover at night. The structure could even be detached from the house. Use a blower to circulate heat only when needed, this is a small load to handle for the guarantee of never inducing unwanted cooling load and being able to control heat better. Plus, the fact that the V in HVAC can be seen as a separate system and is still needed, required, even with radiant heating / cooling. The trombe structure could last 'indefinitely' and has a lower upfront and amortized cost than PV too. It could also be used as a preheater and slightly reduce PV need for hot water but pumping water may in fact come at a net loss with additional complexity for hot water needs... then again with such an 'oversized' trombe structure one may get enough hot water, being only a small fraction of the heat. It should be good for adsorption chilling too, maybe 15kW or so heating capacity could give 2 ton ?? of cooling. The majority of the south facing roof could be the trombe structure instead, or 'trombe roof', reserving a smaller space for PV. And, no 'shading' issues with trombe structure for those around trees.
Dry soil has the same specific heat as sand (19% that of water), and ~90% that of concrete. Wetting the soil (mud) will get you about 3x increase over dry.... one should be able to seal the moisture and even embed a soaker hose to easily 'recharge' if/when needed too. I wonder if dirt could be used as the sorbent media for adsorption chilling, instead of silica gel.
http://www.engineeringtoolbox.com/specific-heat-capacity-d_391.html
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Electrodacus - I can see how 'stepped shunting' the non battery reserve as heat is very attractive : ) That's exactly what you have implemented right? I would argue MPPT only makes sense with dc-dc conversion, the effect is very similar, though it's confusing calling it MPPT. More appropriately you could call it "MPPS" or maximum power point shunting. There is what's called shunt MPPT to I am finding.
I had an idea once to concentrate PV and block / modulate maximum light so as to keep the cell operating at rated 1x power over a wide range of angles / shading / etc.... it is sort of like tracking but you can see how this could be better understood as shunting. -
I had reservation it seemed dubious, however some are advertising PWM + MPPT, which is maybe a little more dubious.
Why are other MPPTs based on MCUs not 'digital'? Can you safely assert all other MPPTs only use linear IC's?
Sorry you do not understand how the Digital MPPT works.
There are two parts to the DMPPT450
First one will take energy from a large PV array and using the so called Digital MPPT method delivered as heat to a set of 6 different value resistive elements.
The Load is made variable by combination of the 6 outputs so that Load can be varied in up to 63 distinct levels (number of combination with 6 ON/OFF outputs is 2^6 = 64 including all off).
By the fact that Load can be varied with this 63 distinct steps allows to maximize the power point for the PV array.
This sort of method only works for resistive type heating and is not applicable to battery charging.
Second part of the DMPPT450 has an output for the SBMS that will do battery charging so battery is also charged from this large PV heating array.
The heating array is split in 6 separate sub-arrays of different sizes and the DMPPT450 can select any number of this 6 separate PV array and redirect them to the output used for battery charging.
Since the total output of this large PV array is much higher than what the small Lithium battery can safely take the DMPPT450 will limit the output to battery at say 50A for a 200Ah battery and will select just a few panels to redirect to battery charging when is sunny and more panels when is less sunny up to all panels if is completely overcast and all panels put out no more than 50A.
So the PV panels redirection to battery charging uses a similar method to the Digital MPPT for the resistive load output.
All energy that is not needed for Lithium battery charging will go to thermal mass storage that has a much larger storage capacity than Lithium.
To better understand all the benefits of this check the long youtube video I posted explaining all this in more details.
What you know as a MPPT used in Lead Acid battery charging is composed of two parts one is usually software in a micro controller and second is the DC-DC converter that is controlled by that software.
The DMPPT450 has the software part (a bit different) but dose not need the expensive and unreliable DC-DC converter.
That DC-DC converter is expensive since it needs large inductors and electrolytic capacitors. Usually is a step don DC-DC converter that is controlled by the micro-controller in order to maximized the charge current to battery by finding the max power point. The current can be varied in a linear way (almost linear in any case with much better resolution) with the DC-DC converter.
A DC-DC converter that will be able to support a 14kW PV array as the DMPPT450 will be extremely heavy (expensive shipping) cost at least a few thousand dollar even for a less than good quality one and have huge internal losses thus probably need active cooling in order not to be even more heavy.
Say you get an excellent efficiency of 94% for the DC-DC converter that supports a 14kW PV array then the amount of heat generated inside the DC DC converter and that needs to be removed will be 14kW x 0.06 = 840W so it will be at full load a heater by itself
The DMPPT450 will have a total TDP of just around 50 to 60W at full load thus passive cooling is simple and the weight is maybe 20 to 50x lower so shipping cost is also significantly lower. Shipping alone from Canada for a heavy DC-DC converter can be more than the price of DMPPT450
The idea is that what you think when you say MPPT is extremely different from a Digital MPPT and while they both maximize the PV array output it is done in a completely different way with different HW and SWGood point. I'm sure all have researched trombe wall, here is one report on efficiency:
http://www.nrel.gov/docs/fy04osti/36277.pdf
That claims nominal 13% efficiency for simple non-insulated design.
What about a fully insulated trombe wall 'structure'. Three sides and bottom have insulation, and the south has double-pane glass with an insulated rolling curtain to cover at night. The structure could even be detached from the house. Use a blower to circulate heat only when needed, this is a small load to handle for the guarantee of never inducing unwanted cooling load and being able to control heat better. Plus, the fact that the V in HVAC can be seen as a separate system and is still needed, required, even with radiant heating / cooling. The trombe structure could last 'indefinitely' and has a lower upfront and amortized cost than PV too. It could also be used as a preheater and slightly reduce PV need for hot water but pumping water may in fact come at a net loss with additional complexity for hot water needs... then again with such an 'oversized' trombe structure one may get enough hot water, being only a small fraction of the heat. It should be good for adsorption chilling too, maybe 15kW or so heating capacity could give 2 ton ?? of cooling. The majority of the south facing roof could be the trombe structure instead, or 'trombe roof', reserving a smaller space for PV. And, no 'shading' issues with trombe structure for those around trees.
That sot of thermal storage and solar heating hybrid has little to do with thermal storage that I'm talking about.
That thermal mass has little isolation from outside since is also used as a collector and is not that different than having large windows for direct solar gain.
Both of those methods are obsolete when compared with PV solar.
The problem is that in clod climate a lot of the energy gained during the day will escape during the night and in cloudy days and so that net gain of that system is extremely low sometimes even negative meaning that is better not to have that installed at all.
They conducted the test in a moderate location not as cold as mine else they will have negative results and that way they get just 13% efficiency but that is not to be confused with thermal storage efficiency and that is the amount of energy delivered by the sun say 5kWh per day per square meter in average that ended up contributing to house heating and in that case it will be 5kWh x 0.13 = 0.65kWh per square meter and day in average over the entire heating season.
A better performance can be achieved with the same surface made of PV panels (17% efficient) and a much smaller thermal mass (about 8x smaller) present inside the house not behind a simple glass with little air gap that has an incredibly low thermal insulation value.
Whit PV panels there is no loss during the night as it is with large windows used for solar gain or that method presented in that paper.Dry soil has the same specific heat as sand (19% that of water), and ~90% that of concrete. Wetting the soil (mud) will get you about 3x increase over dry.... one should be able to seal the moisture and even embed a soaker hose to easily 'recharge' if/when needed too. I wonder if dirt could be used as the sorbent media for adsorption chilling, instead of silica gel.
http://www.engineeringtoolbox.com/specific-heat-capacity-d_391.html
You can see different thermal storage options in my paper. Sand and concrete have about the same storage capacity and compared to water they have almost exactly half the storage capacity by volume.
Capacity by volume is usually important and water can have energy density similar to that of Lead Acid battery (as I mention at some point in my video).
If you want an even better energy density by volume than water then you can use the latent heat capacity of phase change materials with one of the popular ones being paraffin for low temperature storage application.
Absorption chillers can be used with almost any heat source including PV and DMPPT combination and while they are not particularly efficient COP of 0.7 to 1 they can be a solution since the energy source is extremely inexpensive with PV. I still prefer peltier even with COP of 1.4 at 20C delta. Luckily for need I do not need space cooling at my location just heating. Electrodacus - I can see how 'stepped shunting' the non battery reserve as heat is very attractive : ) That's exactly what you have implemented, it is very easy to understand with just those two words no? I would argue MPPT only makes sense with dc-dc conversion, the effect is very similar, though it's confusing calling it MPPT. More appropriately you could call it "MPPS" or maximum power point shunting.
See the answer above also. Not sure stepped shunting is a better definition than Digital MPPT but I'm glad you got the idea and it was simple to understand.
What I do is Maximum Power Point Tracking is just that is in discrete steps 31 or 63 max steps so I call that Digital Max Power Point Tracking.
A DC-DC converter can be used even with a single resistive element to get the same effect but efficiency will be lower and cost much higher.
MPPT with DC-DC converter used in offgrid battery charging is a completely useless technology and obsolete. I made a youtube video about that just search for MPPT obsolete and it will be the first video.
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Here's supposedly an upgraded CMTP02 I linked earlier:
https://www.aliexpress.com/item/High-tech-30A-12V-24V-Auto-Switch-MPPT-Solar-Panel-Battery-Regulator-Charge-Controller/32756101101.html
That listing is very explicit it is supposed to be a 'digital' MPPT and that indeed there is an older version very similar that is only PWM. However for whatever reason MPPT is reported non-working:
I agree with a lot of what you are saying. Still I'm reevaluating oversized solar-thermal + PV, PV alone will [eventually?] be less expensive and reliable, I'll have to see again the price / watt you calculated for this. Adsorbtion chilling can be done via resistive heating too and is a much better option than current TEC, I'd have to consider if AC is still better or not to that, you might as well shunt all the excess into usable cooling if adsorption units are more reliable and more economical. -
Here's supposedly an upgraded CMTP02 I linked earlier:
https://www.aliexpress.com/item/High-tech-30A-12V-24V-Auto-Switch-MPPT-Solar-Panel-Battery-Regulator-Charge-Controller/32756101101.html
I agree with a lot of what you are saying. Still I'm reevaluating oversized solar-thermal + PV, PV alone will [eventually?] be less expensive and reliable, I'll have to see again the price / watt you calculated for this. Adsorbtion chilling can be done via resistive heating too and is a much better option than current TEC, I'd have to consider if AC is still better or not to that, you might as well shunt all the excess into usable cooling if adsorption units are more reliable and more economical.
They of course do false advertising. They even show the PCB of that solar charger and is extremely clear that is a PWM only charger since there is no DC-DC converter in there.
Here is a photo of a proper 60A MPPT solar charge controller with 95% of what you see there being part of the DC-DC converter
The above image is from a clone of the popular Outback 60A MPPT charge controller and that costs about $500
Since PV array is normally that large to provide electricity and heating the cooling will not be a problem with almost any cooling option so absorption chilling low COP is not an issue when you have that amount of excess energy anyway.
I did not look almost at all at cooling since as mentioned before I do not need that. But my personal biased preference is for peltier Not even sure what is the cost and availability of absorption type chillers.
I know absorption chillers are used in RV industry since they can work with propane as the heat source.