PV vs Evacuated solar if you were curious

I see some are unfamiliar with solar evacuated. I thought I’d post a couple of graphs showing the production difference…

Some of the main advantages of solar thermals over PV:

Price: price per kW is significantly cheaper. You can buy 1kW of solar thermals for less than $300.
Efficiency: in a limited space you can get more energy out of them per square metre, 2-4 times more than PV
Storage systems: much cheaper and very easy to maintain.

Here’s the solar production of my ~3kW set from yesterday:

From solar thermals, orientation SSE

Here is the solar production of my ~3kW of solar panels, orientation 120 degree arc pattern

Here is a graph of my heatpump cycle for the last 24 hours under current conditions ( -10°C during the day, -20°C at night). My heat buffer tanks and heat pump do domestic hot water and heating for my house.

At current weather condition of sunny and average winter temps of ~ -15°C to make all my hot water needs and house hold heating, I used 5 kW in a day from from heat pump, the rest was created by my evacuated solar panels, which probably equates to about 87 - 94% less energy than a house of equal size construction for those heating purposes.

If you want more info, you can view my blog that I quickly set up yesterday to give more detail on my projects https://sash991.wixsite.com/mysite/blog


This is great, thanks for sharing! and nice blog too!

interesting to see the graph of output of the solar thermal vs solarpv. I see the solar thermal cuts off about 14:30, what was the reason for that?

I have read that it can be cheaper now to install an additional area of solar PV to provide hot water heating than installing both solar PV and solar hot water separately. A heatpump could also be used to multiply the solar pv output, providing 2-3x the heat otherwise achieved with an immersion heater and standard solar diversion. Id be interested to hear your thoughts on these configurations?

Fascinating to hear about your 1000+ acre organic farm and see the picture of your house where you can just make out your solar hot water system too!


the reason it cuts off at 14:30 is the orientation of that particular set. they point SSE. so by that time the sun is past the point where it can produce enough usable energy. but as the day get longer and the sun get higher it usage period extends to about 17;30 but also during that time period it comes on at 5:30 . but you will notice the operational time is reasonable at currently + 5 hours which is not terribly different then the normal 6 hrs one would expect if it was facing due south at this time of year . also the 5hrs limit is caused by the extremely low night time temps (-20c) so it takes a bit of time to heat those panels and fluid back up to operating temperature of + 56C.

sure PV panels have dropped in price to where it almost comparable to thermals ( currently I can get PV at $0.33 a watt if I buy bulk- but if I buy bulk evacuated solar i will pay about <$200 for 1kw unit). but take into consideration space required a PV panel on average are +16% efficient. thermals are +87%. the space require to install 7kw of PV panels is considerable larger then 7 kw of thermal panels. then depending on configuration --if Net metering is not applicable – then complexity would be another issue especially under lower light conditions it would have to be series of staged compressors to be able to work under such a wide variable power output range . and yet produce less heat energy then if you just used a simple thermal panel. so I do not think the cost effectiveness would be so great in the long run. but sure if you can employ net metering then it would work fine. but problem with that, currently energy companies are backing away from net metering and making it harder to do so or out right banned it. as it seams what the current trend is
then your back to playing with a fine line of either over producing or under producing.

~25 square feet of evacuated solar panel produces about 1kW at peak and maintain it over a long period due to it self tracking design . 25 square feet of PV will produce about 350 W at peak but has a long slow peak and then drops off slowly as it does not self track like the evacuated solar tubes do…

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just mentioning waste heat in the summer time from solar thermal does not have to wasted. there are some systems that convert heat into cooling at 80% efficiency ay low temps of 70 - 80 c

or this one I found is also interesting using lye as an energy storage for heat. i am wonder how much lye i would need to to store 1800 kwh of heat

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Hi Stephen, nice blog, thanks for sharing.

How do you convert thermal to kW / kWh?

I also have a evacuated tubes system for hot water and i’d like to get energy figures out of the temperatures.

sure cab123 - i posted a ino sketch for a flow heat metre here

it does multiple flows meter at the same time but if you only need to plug one in if you like it will work fine …( depending on esp you use you might have to comment out the reference to the second one )

how is your evacuated solar tubes set up. with relief valve, with out relief valve, or open system. the only reason i ask if it with out relief valve you probably have to buy a metal flow meter sensor that handles 2Mpa instead of the more common 1.2 Mpa . when i set up my systems i set them up either as open or with relief valve… though alot of installs are done with out relief valve and the pressure in side there can get pretty high during a power outage or pump failure. the hall effect flow sensors are easily found on ebay amazon or aliexpress when ordering try to buy rated for 100c but lower is fine too if you have the sensor on the cold side…

We also have both. Solar thermal was installed before the PV and when we installed the PV and I built the diverter I thought about how to get the info from the solar thermal into emoncms but gave up pretty fast. Lots of the solar thermal controllers are black boxes, so the only way to get some kind of handle on it would be to use the temperatures in the tank but given that’s also getting PV and there’s draw off for hot water etc. it would be useless.

Our Navitron controller packed up a while back and I got hold of a cheap new one, so just did a quick switch. I probably should have a backup home brew one ready to go in case the new one fails. Solar thermal is a bit more critical than PV. You really don’t want the panels to overheat - not a good scenario.

Anyway, locally today has been a great day, lot’s of sunshine, so plenty of KWhs into the tank from both the solar thermal as well as the PV.

One thing that did interest me though Stephen was your operating temperature of +56C. Our system has effectively a variable operating temperature in that it starts to pump when the temperature in the panels is a set number of degrees above the bottom of the tank, say 10C difference (and stops at say 3C over). As the tank warms, the temperature at which the pump starts to run rises. So there isn’t an operating temperature as such. Not sure how your system works?

Interesting graphs though - thanks for posting.


Hi simon - my +56C reference is to hybrid heatpump/evacuated solar tube design - that I use for heating my house for the winter. the hp pump maintains a base temp of 47C. so the evacuated solar tubes during the winter do not come on until it pass the temperature differential of 10C. in the summer time the evac run as per usual as the heap pump only operated during the winter months.

Hi not sure what you mean by that relief valve, my system is similar to this example:

It has a copper isolated primary circuit filled with water that serves as heat exchanger between the solar collectors and the water deposit. The circulating circuit has a pump that is turned on by a controller that has predefined (configurable) temperature presets.

Last summer just for fun i grabbed 4 DS18B20 and one energy sensor and started monitoring the whole system resulting in this dashboard:

So it looks like a i need a flow sensor in that circulating circuit, i’m going through your sketch, can you please shed some light on this formula?


(flowacount/450.0) / 5 seem to be liquid flow in l/s
dta is the temperature difference

but i’m not quite there yet!


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Aha! I didn’t catch that the operating temperature was essentially a winter starting point.

Hope the weather warms up soon for you!


I’ve got some EV panels (no tubes fitted) on the roof that I have never connected up as I could never find someone willing to do it! I’m thinking of setting up a simple open, drain back system. I need to source a tank though. Interested in the cooling system. I’d thought of connecting up a motorised valve on a Hot Water outlet so if it all got too hot, that could open and dump excess heat down the drain!

The other thing I’d like to know is, do EV systems suffer if they are hot and no fluid is circulating (assuming it has drained back).


450 is the rated flow of your sensor . usually denoted as 0.450 ml per revolution. since I calculated earlier at litre which i was planning to store that info in eprom just never got around to that…

so I would not get confuse later on I left it as 450 then just multiplied by 1000 again to bring it back down to ml
dta = is the temperature differential between the inlet and output side of you tank.
4.188 is the the cal to kw conversion
5 is the sampling rate 5000ms listed above in the ino if you change the sampling frequency you need to change this number as well to match

no they do not suffer too much. depending on the wall thickness of the heat transfer pipe. but they will get terribly hot +250c the problem not so much in the collector it anything after the collector takes a beating…
i like flow back and I used it alot and I would use it alot more but I am having a hard time getting good quality wobble for high lifts . my first batch are great still going just change a brushes every 5 years. it each batch i bought afterwards the diaphrams do not last very long so i went to basic circulating pump again . but they use 3 time the amount of power to do the same work as wobble pump

for me to control over temps i use a flat plate heatexchanger and if it get too hot i circulate it into my ground loop instead of dumping down the drain. but other method is a 3way valve and radiator. depends what cheaper for you dumping down the drain or cooling it off with a radiator or heating your pool or hottube

relief valve if you have a prolong outage or your system stalls for some reason. does it vent the pressure from inside the solar collectors. it would basically have a hotwater relief value near the top of your solar collector. and would vent to the atmosphere or drain back to collection point to recharge the system once it system cools

Yes there is a relief valve on the collectors, as well as expansion vase on both water and primary circuit. Does that affect anything?

I’m still wrapping my head around your work i think is perfectly at reach i’ll shop for a flow meter and play around a bit. Any suggestions?


no your system is fine, it just some installers do not install with relief valve( those systems scare me ) so the pressure get pretty high in those setups. when it stalls . you should be safe with a standard 1.2mpa flow sensor .
pretty much all the ones you find on ebay, amazon or aliexpress should be fine just look at the maximum flow rate of your pump. and buy one that rated ~ 20% bigger it better to be over spec then under spec. and do not buy clear plastic ones

when it comes to error correcting your temp sensors - put them in a wet rag place close together and not touching then error correct so they sit as close as possible to zero for a differential … the ds order is based on thier mac the lowest number will be number one always you might have to try a couple different pairs until you get nice matching ones as some bounce around like crazy

Interesting observation Stephen. I’ve always seen that my sensors come up in the same order after a power outage and had put it down to the topology.

But if it’s down to the mac addresses that is a great insight. There was a discussion on the old forum about this 18 months or so ago and some folks were saying you should id each sensor so you are sure. I’d relied on my observations and what I thought was dumb luck/topology but maybe this bit of info will make everyone’s lives a bit easier.


Brian, there’s mountains of info on the Navitron forum about solar thermal. They also sell tanks - Gledhill from memory.

I think a simple drain back let’s you off the issues around high pressures which can occur in a normal system if the pump fails or the controller controlling the pump fails. Hence my hurried replacement of the controller when ours died. Fortunately it wasn’t sunny that day and I could collect one from the supplier.


Not really, i believe.

There is a deterministic algorithm to find DS18B20 on a network, and each sensor will appear in the same position, however is not related to having a lower or higher device address.

With that said, you will have no luck in predicting the position of a new DS18B20 just by looking at the device address, it’s a bit more complicated than that.

When i set up my DS18B20 network i found that the most reliable way was to make sure each position has a unique device address. Before that, and relying only on the deterministic order they are found, i experienced unrealiable operation, sometimes a sensor not detected at boot ou worse a sensor detected twice. This ruins the data immediately.

Here is how i handle it: After my energy node is running i find DS18B20 and then copy-paste them to the desired positions that are saved on the EEPROM of my chip:

The regular way of gathering the device addresses with one sketch and inserting them directly on the final arduino sketch has the same effect and provides reliable operation.

I wish! If it is over heating there is little chance I need the heat and I am not on a water meter though I don’t like doing it.