thermal sensor location on large array?

Interceptor

I'm installing 16 3'x8' Solaron solar thermal collectors. Due to space restrictions they will be mounted in two rows of 8, one row directly above the other, so the overall array will be 24' wide and 16' tall. The panels have internal headers with a single inlet/outlet on the bottom/top. An external supply header will run along the bottom of the first row, and each panel in the first row connected in series to the panel above it, and an external return header across the top. Although not ideal, this series/parallel arrangement is one of the recommended piping layouts in the manufacturers documentation.

I will be using a Tekmar 157 controller. I'm wondering if a single thermal sensor located in a collector on the top row will be sufficient, or if I should use four sensors in a series/parallel configuration to get an average.  There will be some partial shading of the array in early morning and late afternoon, so I was thinking that an average reading across the top row of collectors may provide better performance during these times. Would this be overkill or would there be enough benefit to make the added cost of 3 extra sensors and wiring worthwhile?

Mark Eatherton

I vote overkill...

I think you're over thinking it. It's going to short cycle 2 or 3 times on start up until insolation gets strong enough to keep the pump running.



One thing you can do to alleviate short cycling is to open the back of the collector, and place the sensor on the back of the absorber, between two risers. This will allow the controller to see the "potential" solar and help in eliminating the daily short cycle dance.



The use of the tekmar 157 goes a long ways towards this goal, but if the sensor is still bonded to the collector outlet, it will still short cycle.



ME

Interceptor

Shading

Mark, thanks for your input. Short cycling is not my concern. Here's why I think it's a good idea to take the average of four (or more) sensors. Take a look at the attached  picture of the roof where the collectors will be installed. If a single sensor is located in the top right collector it will not get an accurate reading of the entire system return temp. The system would start up and continue to run because the sensor is on a hot collector, even though the water is being cooled because >50% of the collectors are shaded.

I realize my site is not ideal for solar. I am prepared to use timers to prevent it from running during certain times, but I also want to get the most out of it as possible. The tree that's causing the shading in the picture will be removed, but there will be others that get in the way as the sun falls lower in the winter months, when the heat is really needed. Of course there will be no leaves on the trees then, which will be greatly helpful.

SWEI

can you spli the system?

Run separate feeds & returns for east and west halves of the array, and only pump when a section gets hot?

Interceptor

ST zoning?

I'd never thought about that. It would be a shame to let ~200sf of hot collectors go unused just because the other 200sf is shaded. I could easily divide it into two or four zones and pipe each one separately, but that adds a lot of cost and complexity on the pumping and control end. Multiple controllers and pumps, as well as the extra piping and roof penetrations. I'd like to keep it fairly simple for now and see how it works for a season, but I also don't want to have to take collectors down later to install sensors.

The fact is I'll need to wait until winter to see what the shading is like when the sun is lower and the trees are bare. Nearly all of my load is winter space heating. The site survey last winter was pretty good, with just a few distant bare treetops getting in the way throughout the day, and more patchy shading morning and evening.

hot_rod

the new solar controllers

most of the digital type solar differential controllers have an East/ West function that uses two sensors on the array. You would need two pumps, one for each array, but the control would know when either array has enough energy to add to the system, and when both arrays meet the delta T, then both pumps run.



hr

Interceptor

control

HR, I've already purchased the Tekmar 157, but if there's a controller out there that can do zoning and multiple sensors I'd like to take a look at it. Who makes them?



I've have four Tekmar 085 solar sensors to install in four different collectors. I also purchased 16 less expensive sensors so I can install one in every collector for possible future use. Whether I decide to do some kind of zoning in the future or even just collect data, it will be nice to have the sensors already in place.



Here's another shot of the roof during part of the day that will cause problems. This would cause continuous short cycling, and other than using a timer to prevent the system from running during this time, I can't think of any way to correct it. The good thing is this only occurs late in the day during summer, when the tank will likely already be at high limit.