PV Gap

Jean Said:

The venturi flowmeter shown below is used to measure the flow rate of water in a solar collector system.?

We Answered:

This is, of course, a Bernoulli's principle problem.
Bernoulli's equation states that P + ?v^2/2 + ?gh remains constant.
P is indicated by manometer height: P = ?gh; thus if we lump P into ?gh the equation becomes ?v^2/2 + ?gh = constant, or
?v1^2/2 + ?gh1 = ?v2^2/2 + ?gh2 (1)
We solve for v1 to find the volume flow rate.
From (1), v1^2-v2^2 = -2g?h where ?h=h1-h2
Because of continuity (constancy of volume flow rate due to incompressibility), v2/v1 = area ratio A1/A2, thus
v1^2-v2^2 = v1^2(1-(A1/A2)^2)
v1^2 = -2g?h/(1-(A1/A2)^2) = 2g?h/((A1/A2)^2-1)
v1 = sqrt(2g?h/((A1/A2)^2-1))
Then volume flow rate dV/dt = v1A1
To keep things simple we equate the 1.4 cm deltaH of oil to the equivalent value for water = 0.76*0.014 = 0.01064 m.
The answer is 1.5258E-5 m^3/s.
If you check this using the calculator in ref. 1, you will need to input 1.3214E-5 m^3/s (13.214 cm^3/s) because they use a factor of 4/3 to multiply the v^2 terms, which is explained in ref. 2.

Bonnie Said:

Is an evacuated tube solar collector worth the $5,000 it costs to buy the system and have it installed?

We Answered:

Depends on where you live, and what kind of water heater it will be displacing / assisting. Sounds like you have a location that will freeze sometimes, but where are you?

Edit: I don't know about Connecticut. If you're displacing an electric water heater, you have a chance of making your money back. Something like 1/3 of a house's electric bill can go into heating water (unless you have electric heat, too). And the evacuated tubes will work in the winter, but you would probably need a backup heater unless you severely oversized your solar heater. And you have to knock the snow off the panels, if that's a problem in your area.

Maybe your best move is to contact a local solar installer and ask for a nearby reference. Then ask the reference how they like it. If there is not nearby reference, that's a clue that solar might not be a good deal in your area.

Stacey Said:

TROMBE WALL- Questions about this type of solar collector?

We Answered:

It's a passive system that only works within relatively limited temperature bands, with constant sun, has poor efficiency and that can not store relevant amounts of heat for most locations.

It is, on the other hand, a pretty good looking architectural solution that won't need repairs and that can't break (unless someone breaks the windows...).

If I had to build a system myself, I would go for a modern active system which uses high temperature collectors and probably either a water tank, or even better, sand or concrete as the storage medium.

Greg Said:

TROMBE WALL- Questions abvout this type of solar collector?

We Answered:

A Trombe wall system is essentially a shallow greenhouse with a back wall and floor of solar heat absorbing material such as concrete or stone. These systems are very good at providing heat to a structure not only in the day but also at night because of the thermal mass stored in the stone wall and floor during the day and given off during the night. I've also seen systems where a small solar collector was placed in the space to provide hot water, which work very well because they're not exposed to cold outside temps. I don't know of any cons for these except maybe for the aesthetic design aspects, which could be dealt with.