RSS
Home
The Other Solar Power
Print
Email
RSS
Subscribe
.jpg)
Thermal Solar Panel
Converting sunlight into electricity is quite a technical feat, but there is nothing particularly magical about using the sun to make things hot. Thermal solar panels don’t have the sizzle of their photovoltaic cousins, and in many cases they have a negative reputation from the 70s. But if you have a hot water heater using fossil fuels – you need to give Thermal Solar another fresh look. I recently implemented a thermal solar system at my home. The technology is relatively primitive compared to the PV alternative – but this isn’t about technology. It’s about hot water.
My thermal system went up alongside my PV system. But other than utilizing the sun’s power – they have nothing in common. I’ll get to the design and choices below, but first a little about the experience compared to PV.
PV was very analytical. Any engineering type would love the PV design process. It is all numbers based. They figure out your average sunshine based on your solar view (trees, buildings, etc.), angle, and latitude and longitudes. From various almanacs and tables, they can determine the average production of a proposed PV system. They can then compare that against current usage from the electricity bills and maximize the trade-offs to determine the performance of a proposed system. How much it will generate, and based on some assumptions around electricity price increases, rebates and incentives, and household usage - they determine the payback period.
Thermal solar can’t do that. Sure a hot day will result with a tank of hot water, but it only counts if you will use the hot water. Most systems utilize a fossil fuel backup – so if all the demands are placed on cold days or at night, then the solar benefits are unrealized. This isn’t true for PV systems interconnected to the grid – here the electrical meter runs backwards during the day and forward at night – average generation and usage is what determines effectiveness.
Making matters worse, I met with three thermal system professionals and got five opinions. There are lots of variables in thermal hot water besides the ROI. To avoid freezing, which is better Glycol or drain-back? Which type of panel- traditional or evacuated tube? How many panels? The problem was for the most part, these people were either plumbers or salesmen, not both. So without a clear ROI, nor a clear design –things were very frustrating. The one trusted advisor I had was the house plumber, but his only solar experience was tearing out abandoned systems from the 1970s. He couldn’t fathom why anyone would want thermal solar – especially since he had such nice shiny new boilers to sell me.
In the end, I got a three panel drain-back system that provides first my domestic hot water(DHW) – and then it complements my hot water heating system. There are three tanks in total; a small drain back system, an 80 gallon two exchange domestic hot water tank, and an 80 gallon store for the in-house hot water heat. A simple computer controller determines by various temperature readings when to turn the system on or off and which tank to heat.
The initial design was for DHW with only two panels. The tank has two heat exchanges – the lower one uses solar heat. Since heat rises, it could heat the entire tank. However, if it can’t keep up or the sun is gone, the upper exchange is heated by the natural gas boiler. This is fairly efficient since it can’t really heat the whole tank.
We added a third panel and a second take to supplement the heating system. When the DHW isn’t being used much, it doesn’t take long for the tank to get hot (especially with three panels). So instead of shutting down, it flips priority to the heating system. The house has in-floor radiant heat which loops through the house, but before it goes back to the boiler it runs through this tank which either heats the water warm enough (110) or prewarms it before entering the boiler. This is impactful particularly in the Spring and Fall when we get lots of sunshine during cool days. Three panels is very effective at heating either tank pretty quickly.
Drain-back is simpler and requires less maintenance than a glycol system. It has two drawbacks. The solution must use gravity to drain the pipes – so the panels have to be reasonably directly above the tanks (check) and the initial energy required of the pump to fill the pipes is greater than just turning on a pump with the pipes filled. I opted to cancel-out the second factor based on the PV system and the cost of Glycol. Drain-back systems require little to no maintenance.
Evacuated tubes do get the water hotter under certain conditions – but the firm I selected strongly encouraged traditional panels based on their experience. I did find a study performed by the local university that concluded traditional panels were more effective in the Rocky Mountain climate.
I don’t really know how effective my system is, or what the ROI will be. It really depends on too many factors that can’t be measured. I can’t compare to prior records because the remodel changed too many factors including insulation, windows, and square-footage. But I am comfortable and confident the system Is effective. I have various measures and indicators telling me the system is running as designed.
The point of this is thermal energy really makes intuitive sense. But the popularity of PV systems, the complexity and fuzziness of thermal, and the caliber of “professionals” out there – makes the project a daunting task. They work in surprisingly low temperatures, and they are not as sensitive to partial shading as PV panels. Thermal solar systems are very efficient - more so than tankless hot water heaters. Or you could combine them and use the solar system to pre-warm the water and a tankless system to make it hot. Every time you get into a hot car, think about how simple thermal systems make things hot.
Related Articles:
Comments
