Water loss from solar system or temp increase?

[AUSpool]

I'm almost certain this is not the right place for this thread/question but it's leading from the bellow post from Matt, JoyfulNoise. I've seen that link twice now but am having a hard time applying the evaporation from water surfaces equation to my application. Evaporation is a function of the pool water temp but I can't figure where to input that value. I want to change the pool water temp leaving all other values the same.

It all depends on the air temperature and relative humidity versus the water temperature. If air and water temperatures are close, and the air is not too dry, evaporation will be minimal. You can start at this article over at The Engineering ToolBox to get a feel for what's involved -

Evaporation from Water Surfaces

I suspect I have a leak in my solar heating system. The water loss increases noticeably when it's on but is it a leak or just an increase in water temp? Over 14 days I calculated my evaporation rate at 17L/hour (4.5G/hour) @ a water temp of 20oC (68oF) with the solar system off. After turning it on, over the next 22days I calculated my evaporation at 29.5L/hour (7.8G/hour) @ a water temp of 26oC (79oF). I calculated my water loss by timing my water additions and using my taps flow rate of 2.4L/sec.

Our weather has improved over the last two months, summer is comming and it is a bit of an assumption to assume all other factors have remained the same but I don't think it has changed significantly. Any help would be greatly appreciated as if I do have a leak it must be underground and that's going to be a big job to dig it all up.

 

[JoyfulNoise]

Temperature comes into play through the two parameters -

x_s = humidity ratio saturated air at the same temperature as the water surface (kg/kg) (kg H2O in kg Dry Air)

x = humidity ratio air (kg/kg) (kg H2O in kg Dry Air)

Unfortunately the website just gives you lookup tables for the values. The x_s parameter is the one where your water temperature comes into play because it assumes that the air directly above the water surface (in an infinitesimal sense) is fully saturated with water vapor at the same temperature as the pool water. The x parameter is simply related to the relative humidity of the air above the pool as measured by a wet-bulb thermometer. You can probably reproduce those curves in Excel to get more exact values for your specific temperature values.

If you suspect your solar heating system is leaking, is it possible to just inspect it while it is running? Is it a roof top system? Is access to the roof difficult? Doing the exact calculations can prove to be quite difficult to match to water loss because of shifting wind patterns and changes in temperature. If one really wanted to examine water loss, you can build your own evapotranspiration devices -

http://swhydro.arizona.edu/archive/V7_N1/feature3.pdf

EVAPORATION AND ITS MEASUREMENT

 

[mas985]

I'm almost certain this is not the right place for this thread/question but it's leading from the bellow post from Matt, JoyfulNoise. I've seen that link twice now but am having a hard time applying the evaporation from water surfaces equation to my application. Evaporation is a function of the pool water temp but I can't figure where to input that value. I want to change the pool water temp leaving all other values the same.

If interested, there is a link in my signature for a heat transfer spreadsheet. It gives you a bit more control over your situation and uses data from a local Wunderground weather station (even some in AUS just use "town name", AUS) so you can get more accurate weather inputs. It will also give you evaporation rates.

 

[xyz]

Run the solar system at night. If it is leaking, you will see a loss. If it is temp related, you will see less loss than a typical night.

 

[AUSpool]

Thanks for the replies, it's much appreciated.

Matt, thanks. I was able to find 3 places to add a variance for the water temp a few weeks back when I saw the first link. I'll go back and use the Xs values and compare them to my actual, adding 10% for error and see how they compare.

Access to the roof is good, I've been up there and no obvious leaks. Which leaves about 20' of underground pipes - not happy about that. I did make a mental note to myself that I should check it all before backfilling.

Thanks Mark, I have followed the links from your sig in the past - very informative and much appreciated. I'll have to dig out the old lap top to use excel when I get the chance.

Thanks Jon, a good idea and I'll give it a go although I think it may be difficult to accurately measure the loss over night. I have been messuring the loss of 3-7days so far.

 

[xyz]

OK, so run the solar 3 nights and NOT 3 days in a row. But then you may not like the temp of your pool! ;-)

 

[AUSpool]

^Yes thanks, I think that might work and now is the perfect time, we're getting days in the low 30's but nights are still dropping to 15, oC that is.

 

[AUSpool]

Matt, thank you, :bowdown:

I finally got some spare time to crunch some numbers.
My actual values for evaporation are;
17L/hour @ 20oC over 14 days with the solar system off and no allowance for rain.
and, 29L/hour @ 26oC over 22days with the solar system on and no allowance for rain.

From Evaporation from Water Surfaces

gh = θ X A X (Xs - X)

Where;
gh = evaporated water per hour (kg/hour) - l'm going to call it L/hour.
θ = evaporation coefficient (25 - 19v) (kg/m2/hour).
v = velocity at water surface (m/sec) - 20km/hour adverage = 5.5m/s.
A = surface area - 24m2.
X = humidity ratio of air (kg/kg) - 0.0098kg/kg @ 20oC & 50% relative humidity.
Xs = 0.015(kg/kg) @ 20oC, 0.02 @ 25oC, 0.025 @ 28oC & 0.027 @ 30oC.

Xs values are from http://www.engineeringtoolbox.com/humidity-ratio-air-d_686.html

Crunching the numbers I get gh = 16L/hour @ 20oC and 46L/hour @ 28oC.

Running the numbers through the online calculator for v= 5.5, A = 24 and the previous Xs values I get;
16L/hour @ 20oC water temp,
32L/hour @ 25oC water temp,
47L/hour @ 28oC water temp,
53L/hour @ 30oC water temp.

Given the variables I'd say that's no leak.

I haven't had a chance to run the solar system at night but will give it a go.

I had noticed water in our storm water outlet at the street and had thought it could be ground water from a leak but it had a salt reading of ~230ppm so not pool water which is up around 6000ppm.

As an added bonus I can use my evaporation values as a starting point for an auto topup system.

Does anyone have any evaporation data they wish to share?

 

[JoyfulNoise]

Steve,

A good place to start with evaporation is to look at Class A Pan Evaporation data for your region - Water and the Land: Evaporation

 

[AUSpool]

Just when I thought I had the basics of hydrolic engineering...

Now for pan evaporation were using millimetres per day. From the data from around South East Queensland I get 4mm/day annually or 6mm/day for summer - seems low. With an adverage of 410L/day @ 20oC that's 17.5mm/day and at 668L/day @ 26oC that's 28mm/day. Now I think I have a leak with the solar on or off.

I might have to enlist the neighbours into supplying data to compare.

 

[JoyfulNoise]

6mm/day is ~ 1/4"/day. I think that's a reasonable figure over here in the US during the summer swim season...

- - - Updated - - -

Have you ever done a bucket test?

 

[AUSpool]

I was going to mention that it is time for the bucket test.

It's odd that my calculated evaporation does closely match my actual evaporation. I had thought that both do appear high but we are exposed from the northeast to southeast which covers most of our prevailing wind. The weather reports consistantly predict 15 - 25 - 35 Km/hour wind which is why I chose V = 5.5m/s

Going back to the evaporation calculator, at V = 0.5m/s evaporation = 8mm/day and at V = 5.5m/s evaporation = 32mm/day which kind of explains the difference.

 

[AUSpool]

The bucket test was a challenge. I put a bucket of water on the step in the morning and came back in the afternoon to find it floating over the other side with a pool toy in it. So I filled the pool up to my mark in the skimmer and replaced the bucket with some rocks in it. In the late afternoon the pool water level had droped ~12mm and the levels inside and outside the bucket still matched.

I'm scaring myself now, I've added 22,000L to the pool over the last 35 days, another two days worth and I would have exchanged the pools entire volume. If I did have a leak I would would be leaking salt water but my salt level is still bang on 4600ppm and hasn't changed over the last month. I'll just have to suck it up and pay the water bills.

 

[JoyfulNoise]

...I'll just have to suck it up and pay the water bills.

Isn't living in an arid climate SO. MUCH. FUN!

In Tucson we have 90" to 100" of evaporation per year with less than 11" of rainfall on average. So, in the course of 18 months, I pretty much evaporate my entire pool volume away. I once thought about putting a high quality water meter on my auto fill line to track fill water usage. Then I thought to myself, "why would I want to spend over $100 USD on something that will only make me more depressed?"