System Design: Solar Thermal Power Generation

Tryden

I was recommended this site as a way of getting help on the last leg of my design journey.
I've hit a wall in my engineering ability (as a structures guy) when it comes to thermal dynamics.

In short, I want to replace a custom heat pump's compressor with a proprietary expander tied to a generator, and use Solar Thermal as the heat source to drive the temperature delta thereby generating electricity.

I'm challenged with sizing the component of the whole system in order to build a demonstrator - additionally, I know there are little things in the system design that are needed, but don't know what they are. I am hoping I can get some help.

So here is the concept:

Evacuated Solar Thermal Tubes running dowtherm 10 (or eq.) on a drain back loop into a series of tanks with heating oil. Heating Oil tanks are intended to store enough heat to keep the working fluid @ or above temp for 6-8 hrs. Then there is a loop between the oil tanks and a plate heat exchanger - the loop should stay above 200c for as long as possible. the goal is to generate power at the peak periods of 7-9pm and 6-9am.

Working fluid passes on the other side of the plate heat exchanger and picks up heat (200c+) then pass over the expander - the pressure/phase change driving the expander+generator. The working fluid then passes through a "cooling" plate heat exchanger. To get the bare minimal energy generation, there should be a temperature delta of 30c between the hot and cold sides of the expander. I don't know the pressure drop over the expander - id like help finding out what it needs to be - in order to adjust the mechanical design of the expander. I don't know what the mass flow rate needs to be.

The cooling plate exchanger side has a target of heating water for domestic use - assuming the water should maintain between 100-140F temperature - and assuming a 40ga change over 2x a day (i.e. 2-3 showers a day)

I need to know how many SF of Solar Thermal Tubes I need to achieve the goal of 5Kwh output. I do know that this type of system typically is between 10-12% efficient on energy conversion. If we could get it to 18% efficient - it could be a seed change.

I'd like to be able to figure out: pump sizes, flow rates, thermal storage amounts for the oil tanks, and how much water i need keep in storage (100ga?) and....

Well - by now you can tell I am pretty much out of my depth when it comes to this sort of thing. Help would be appreciated - and ....equations are always appreciated. I just don't know how to calculate between the heat transfers and flow rates....Anyway.

Here is to a new year with better energy possibilities!

- Tryden

Jamie Hall

I'll start you with the collector area, anyway. If we assume, conservatively, that you can get a 10% thermal efficiency out of it (which is, in my opinion, high for the temperatures you mention), and we also assume you are in New England or actually anywhere pretty much east of the Mississippi and north of the Mason-Dixon line, the general rule is that you can get 3 hours of effective collection per day. Some days you'll get more. Some, particularly in the winter, you will get less.

So... crunching all those numbers together with the dear old solar constant, which is conveniently 1 KW per square meter, we find that you can get an average energy output of 0.3 KWh per square meter of effective collector area per day. So your 5 KWh per day energy will take about 17 square meters of net effective area. Note that that's net effective area -- the area which is actually receiving sunshine and conducting, effectively, to the heat transfer medium.

Now you also mention wanting to generate power from the stored energy in the morning. That energy will have to have been stored the previous day (if you are lucky and it isn't cloudy several days in a row, which is often the case in the winter). You mention 6 to 8 hours. That won't cut it. You need an absolute minimum of 24 hours, and anywhere in the northeast more like 72 hours would be more conservative.

Personally I would like to see you also double check your power and energy requirement relative to your load, which you don't mention. Even with a pretty high efficiency heat pump, your 5 KWh energy only rises to about 50,000 BTU, or about 4,000 BTUh over a day of heat demand.

Hot_water_fan

Interesting! Why this over a heat pump + Solar PV?

hot_rod

It seems like a lot of phase changes to generate electricity?

Radiation to heat oil. Oil to power the expander. Expander spins a generator. Pumps to run the solar thermal also. Every time you go through a phase change or heat exchanger you lose some efficiency.
It seems like PV with a battery bank would be much less technology and higher reliability?

Evac tubes in snowy winter climates can be a liability also. Unless they are at a steep angle they don't shed snow as the glass never gets warm.

Some pics from a New England area solar contractor. The entire roof melted off and the tubes still were not performing much. Evac tube arrays are not so easy to rake snow off of either. Tubes are annealed glass, more prone to breakage compared to flat tempered glass.

Tubes after a Montana hail storm.

Tubes when they start losing vacuum

hot_rod

NREL is one place you can drill down on radiation data.
First you would want to define the "load"
then design the array around that number. Regardless if it is PV or thermal.

hot_rod

Give Bill down at Arctic Solar a call. He developed a clever small scale parabolic collector, he may have some ideas for your project

jumper

Hot_water_fan said:

Interesting! Why this over a heat pump + Solar PV?

My question as well. PV is less expensive when it's off grid.