Convert Flow and Temperature Difference to kWh

[Brandon]

thanks guys

I am using this to calculate the kW going into an underfloor zone, as the room seems to not be warming adequately, and they are trying to ascertain whether it is the underfloor or the insulation/drafts that are the issue.

so can i use my equation with stb's alteration of 14.2857?

any further offers?

The UFH is running straight water AFAIK

[KLD]

Brandon, for your application a rule-of-thumb is plenty good enough. Going from 14 to 14.285 is only a 2% change, and your other variables are certainly not known to anything like this accuracy.
Have you measured the floor surface temperature? Or, if that's not too high already, have you tried to increase the flow temp into the UFH loop? How big is the difference between flow and return? If very small, then the floor is apparently not able to shift the heat into the room. Maybe some part / too much is covered by furniture, or a rug?

Good luck!
Klaus

[Brandon]

I will know more tomorrow when I go and make the measurements.

I was going to employ the masking tape on the surface to be read by an IR thermometer to ensure a fair comparison on the floors any objections?

I have a two sensor digital thermometer that has velcro k type sensors on that we use for flow and return temps.

[KLD]

I have a two sensor digital thermometer that has velcro k type sensors on that we use for flow and return temps.

I've got several 1-wire sensors along the solar circuit, and found from the longer-term recorded data that it's actually quite difficult to get a reliable thermal contact between sensor and a surface. Depending on the shape of the probes you've got, and the temperatures involved (pipe as well as ambient), there might be a 2 or 3°C mis-match between actual pipe temperature and what you measure. A few repeat measurements, trying different ways of holding or clamping the probe against the pipes, and careful analysis of what you see should get you  'round those issues!

Klaus

[dhaslam]

How do you measure the flow?   The little gauge on the manifold doesn't give much of a reading.    I used an estimate of 6 watts per metre run of pipe and tried to compare that with the drop in temperature in the buffer tank over a period.   The latter seemed to give a slightly higher  figure.   If may be possible to get some sort of figure from the heat source in your  example as well.     

[hiccup]

Hi

Going back to the formula for power (kW) from flow and temperature difference, I'm trying to determine if the temperature and mix of the fluid makes a significant difference.

The reason is that on my system the displayed power matches my calculations at about 50C but is 10% out at 70C.

I've got the figures for how temperature affects heat capacity of water, but with a 40% propylene glycol mix what is going to be the difference over the normal temperature operation range of solar?

Any ideas?

Hic!

[EccentricAnomaly]

There are separate changes with temperature: specific heat capacity and density.

Googling "Propylene glycol coefficient of expansion" finds:

http://www.mrc-eng.com/Downloads/Brine%20Properties.pdf

Turning that into Python:

Code:

#!/usr/bin/env python2.6
# -*- coding: utf-8 -*-

def p(conc, t, a):
    """ Generic property formula parameterized by a for particular properties.
   
        conc is proportion of glycol in range 0.0 to 1.0.
       
        t is temperature in °C.
    """
    t0 = 273.15
    t += t0
    return a[1] + a[2]*conc + a[3]*t0/t + a[4]*conc*t0/t + a[5]*(t0/t)**2
   
def Cp(conc, t):
    """ Specific heat capacity: kJ·kg⁻¹·K⁻¹ """
    return p(conc, t, (None, 4.47642, 0.60863, -0.71497, -1.93855, 0.47873))
   
def density(conc, t):
    """ Density: kg·m⁻³ """
    return p(conc, t, (None, 508.41109, -182.40820, 965.76507, 280.29104, -472.22510))
   
def volumetricHeatCapacity(conc, t):
    return Cp(conc, t) * density(conc, t)
   
def pr(conc, t):
    """ Print properties for concentration and temperature. """
    print '%3.1f  %2d°C  %6.4f kJ·kg⁻¹·K⁻¹  %8.3f kg·m⁻³ %8.3f kJ·m⁻³·K⁻¹' % (
                    conc, t, Cp(conc, t), density(conc, t), volumetricHeatCapacity(conc, t))

print 'Conc Temp  Spec. Heat Cap.     Density         Vol. Heat Cap.'
for conc in (0.5,):
    for t in (50, 70):
        pr(conc, t)

The actual numbers it gives are a bit odd (e.g., for pure water at around zero degrees
the density and specific heat capacity are a little bit higher than the normally quoted
values).  Still, I assume the general shape of the curve is reasonable.

OK, running it for the temperatures of interest assuming 50% glycol:

Conc Temp  Spec. Heat Cap.     Density         Vol. Heat Cap.
0.5  50°C  3.6991 kJ·kg⁻¹·K⁻¹  1014.605 kg·m⁻³ 3753.160 kJ·m⁻³·K⁻¹
0.5  70°C  3.7434 kJ·kg⁻¹·K⁻¹   998.305 kg·m⁻³ 3737.055 kJ·m⁻³·K⁻¹

The specific heat capacity (heat per kilogram) increases a bit but the
density reduces so these effects partly cancel out.  The result is that the
volumetric heat capacity (heat per cubic metre (i.e., 1000 litres))
reduces a tiny bit.  Much less than 10% so that's not the whole story
by any means.  Drat, or something.

If you're not actually measuring the flow rate but assuming it's constant
for a given pump setting I wonder if a change in viscosity could also be
involved.

[hiccup]

Hi EA

Thank you for the info, I'll look into this a bit more.

I am measuring the flow rate using the Grundfos VFS 1-12 QT vortex flow meter supplied with the TDC4, and trying to calculate the power (and energy) from the flow and return temperatures and flow rate, then trying to get it to correspond to the figures on the display.

The flow sensor is supposed to give a linear output proportional to flow but the info is a bit thin on the ground regarding applications, formulae, and any temperature effects. I've asked Grundfos directly for the info but no response as yet.

Of course there are lots of tolerances that stack up, but nothing that accounts so far for the variation I've seen. Even if the flow meter is not accurately measuring the flow, the numbers I calculate should follow the same curve as the numbers on the TDC4 and so far they don't.

Hic!

[hiccup]

Hi

Just an update on this.

Through my experiments I can now confirm the following:

The TDC4/ether calculates the power using a formula something like

Power (W) = X * flow * temperature difference

as has been suggested in this thread.

X is probably looked up from a table based on glycol percentage. I don't think they compensate for temperature of the fluid.

As has been pointed out:

Power (W) = specific heat capacity (J/kg K) * density (kg/m³) * flow (m³/s) * temperature difference (K)

and both specific heat capacity and density change with temperature but the effects more or less cancel out.

To be accurate you would have to have some data specifically on the propylene glycol + inhibitor mix that is in use.

I took a look at the PDF suggested by EA but this is only an approximation - as EA has said, the numbers are a little strange for pure water.

So I went Googling and fount the datasheet for Tyfocor L which is supposed to be similar to the Navi HTF. By taking the numbers from the 40% curves for density and SHC I've got a composite curve for X  across the 0 to 100C fluid temperature range for my system.

BUT (you just knew this was coming!) the difference it makes to the power calculation is still less than 1% over the typical heating temperature range.

So, where was I seeing a big error - it's the input data of course! 

The TDC4 only gives the flow temperature as integers, and the return temperature (part of the flow sensor) to 0.1C, so the error in the temperature difference can easily be 5 or 10% depending on the actual difference.

I'm fairly certain the TDC4 only uses the same temperature values to perform the calculation as are available via the ethernet protocol (although note that the display of the return temperature is only integers on the unit)

The only way to get better accuracy really is to ignore the temperatures given by the TDC4 and measure them separately.

I'll probably go the whole hog and build a separate energy metering unit using the VFS sensor and the PT1000 sensors.

Hey ho - another project 

Hic!

[Poolguy]

....... or you can use SUNeLog which has these energy calculations integrated in the Software, along with some others. Next to arrive is 'solar effeciency' or the coversion of available isolation to delivered heat. Its an idea to enable the system to be 'tweeked' (I like tweeking).

Andrew

[desperate]

Just to throw a few more spanners in the works........

The tdc3/4 calcs are based on the flow rate in litres/min and the delta temps yes?  How is this measured?

Am I correct in assuming that the flow is worked out with some kind of flow gauge/sight glass? ok fair enough but possibly a bit inaccurate.

Temperature difference, is that the difference between the S1 and S2 readings? if it is the basis of the heat calcs, I dont think they would have any basis in reality, it assumes that the temperature drop across the inlet and outlet of the cylinder coil is the same..........hmmm I dont trust that as far as I can throw a grand piano.

Has anyone tried measuring the difference across the cylinder coil? actually not an easy thing to do as the external pipe temp is quite slow to react to the internal water temp.

I am wondering if it is better to measure the usefull hot water delivered to the house at the outlet of the cylinder.

Desperate

[KLD]

Desperate

Spot on! But to be fair, in the manual they don't claim that the TDC3 provides a properly measured value for the harvested heat, it's more like an indicator only.
The flow rate is not measured at all, but the customer put in whatever is his/her best guess. (The TDC4 comes with an additional flow rate meter, and two extra temperature sensors, completely different league compared to the TDC3.)
The temperature values used by the TDC3 are indeed S1 and S2. I've recently installed sensor pockets in the flow and return pipes next to the TS coil, and it's quite surprising how well the return temp follows the tank temp. Taking S1 as the flow temp into the coil is a bit more adventurous, since that doesn't allow for any pipe losses.

Only this afternoon I put two additional sensor pockets in, next to the panel entry / exit. The readings from clamp-on sensors (1-wire, using Terry clips, burried under the pipe insulation) were really not accurate enough. I'll report back when I have the first useful data 

Klaus

[desperate]

Thanks for the reply Klaus, I am quite interested in this, I am gearing up to go for MCS accreditation and also I will be putting together an advertising brochure, and I dont want to get involved in any spurious claims.

Quote
Taking S1 as the flow temp into the coil is a bit more adventurous, since that doesn't allow for any pipe losses.

And further to that, what exactly is the Sensor in the panel in contact with?? clipped to the manifold and buried in the insulation?? I have measured a 10 degree difference between the S1temp and the outlet pipe temp, measured with a food probe type thermometer in the insulation. OK not exactly scientific, but I do wonder.

Those fittings of yours look good, I keen to hear more.

All the best

Desperate

[desperate]

Evening Klaus et al

Following on from last nights conversation, I carried out a bit more measurement today

First I measured the temps on the outlet pipe of the panel containing S1 sensor approx 60cm from the panel
with S1 reading 71 as the pump turned on the pipe temp read 61.9 and over the next 30 seconds the temp rose to 64.9 and then over the next 60 sec dropped to 62.8 whence the pump stopped. At this time S1 was reading 67

Considering that rise and then fall in temperature measured at the pipe is entirely what I would expect, and the response would indicate that it is a pretty fair representation of the fluid inside,  yes??
But the fact that this pipe temp seems reasonable but 10 deg below the indicated panel temp suggests to me that S1 is not really measuring the fluid temp in the panel.

I,m wondering if my sensor is actually measuring the temp of one of the tube sockets, which is bound to be a little higher than the fluid temp. Also I measured a temp of 57 deg at the coil inlet ,  at the same time  S1 was reading 71.

All of the above ramble I believe casts quite a lot of doubt on the heat output figures based on the TDC3 readings, as this is based on the Delta temps that are usually in the order of say 6-10 degrees a 2 or 3 degree inaccuracy could throw out the calcs by 20-50%, and thats just the S1 error

To calibrate my readings taken with a food temperature probe, I waited untill Mrs Desp was out the way and then applied the scientifically rigorous "Broccili Test"  ok ok I touched the probe on the outside of her steamer pan(full of Broccoli) 100.4 was the reading............good stuff

Please dont take this in any way as a criticism of the gear supplied by Navitron, it isn,t, after all by 5 this afternoon the tank was up at 75 deg c top and bottom and we were dumping heat into the neighbours

It is more an exercise in fizzicks/plumming/loonacy and having to little to occupy the brane


Desperate

[KLD]

Desp,

Reading temperatures off the surface of the pipes is not very accurate. While you were exercising between fissicks and loonatic broccoli, I went up the ladder again to fiddle with the sensor pockets installed the day before. It turned out that the probe wants to be inserted quite deeply into the pipe / manifold before it accurately reads the flow temperature. On the entry into the panels, I initially put quite a short pocket into the elbow, reaching only just through to the panel end of the elbow. After all, I wanted to measure the temperature of the water entering the panel, avoiding the contribution of the first heatpipe. Despite the whole palaver being wrapped in insulation, this probe measured 4°C low at 60°C flow temp. So, up the roof again and put a longer pocket in. After that change, the two sensors in the flow pipe -- one next to the panel, the other next to the thermal store solar coil entry -- are within 0.5°C to each other, as are the two sensors on the return pipe. (This is measured with pump going full pelt, and no solar input.)

Let's see how it behaves when the sun comes out today. 
Klaus