# SWH circulation pumps electricity consumption per hour

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nino
SWH circulation pumps electricity consumption per hour

Hello,
I have two questions related with SWH system circulation pumps:

1) How does SAM calculate the amount of hourly electricity that circulation pumps spend in Solar water heating system?

I checked the official SAM help file and "Technical Manual for the SAM Solar Water Heating Model", but could not find an answer to this question.
Does SAM just use the actual "Pump Power (W)" input as the hourly overall consumption of circulation pumps?

2) If yes, then in which cases are these two pumps considered to be working?
NREL's "Technical Manual for the SAM Solar Water Heating Model" mentions the pump power as one of the parameters in calculation of saved energy (Qsaved):

Qsaved = Qaux,only - Qaux - Ppump

But when does SAM consider the pumps to be working, and when it does not?

Paul Gilman

Hello,

1. The pump power is the product of the rated pump power and pump efficiency that you specify. For a 45 W pump with an efficiency of 0.85, the pump power is 38.25. You can see the hourly pump power in the results.

2. Pump power is consumed during hours that the collector delivers useful energy. In the hourly results, you can see the pump power for time steps that Q useful > 0. When Q useful is zero, the pump power is also zero.

Best regards,
Paul.

nino

Thank you for the reply Paul.

I did not understand you this part:

"you can see the pump power for time steps that Q useful > 0."
What is the "> 0." ?

The "Q useful" you mentioned is the "Q collector to Tank (kW), Hourly" from the SAM 2013 screenshot below:
https://www.dropbox.com/home/stuff?preview=shw_SAM.jpg

?

(I am using the 2013 version because this was the last one which supported my operating system: windows XP).

How does SAM calculate the hourly Pump power (W)?
For example in the upper screenshot, there are hours when the hourly Pump power (W) is 40 (equal to "Circulation Pump power), but also 26.66, 6.66 and so on.

nino

The upper screenshot link is incorrect. I apologize for that.
This is the correct one:
https://www.dropbox.com/s/xg3bblhusse7tpb/shw_SAM.jpg?dl=0

Paul Gilman

Sorry about the '>' -- the website editor misinterpreted the greater than sign that I typed. I've corrected that in my reply above (#2).

My description is for the current version of SAM, which has an improved SWH model. Unfortunately, I do not have access to the code for SAM 2013.1.15, so cannot check the equation there.

In the current version of SAM, the pump operates whenever the collector delivers energy. Its power consumption is constant at the rated capacity and efficiency that you specify on the input page.

In SAM 2015.1.15, the pump operates whenever the collector delivers energy at the rated capacity without an efficiency adjustment. It also operates at lower than its rated capacity when the collector energy is lower than some threshold, but I'm not sure how that power level is calculated.

Best regards,
Paul.

nino

Thank you Paul.
This is exactly what I would like to know:

"It also operates at lower than its rated capacity when the collector energy is lower than some threshold, but I'm not sure how that power level is calculated."

It is really interesting that the "Technical Manual for the SAM Solar Water Heating Model" .pdf does not give an answer to this question.

I know you have not been one of the authors of this one, but I must say that you alone did a magnificent job on the "SAM Photovoltaic Model Technical Reference" unlike mentioned "Technical Manual for the SAM Solar Water Heating Model" paper.

ndiorio

When the pump power is calculated, it is the product of the rated pump capacity and the pump efficiency. The pump power is not applied when there is not useful energy collected or there is not incident radiation present. Useful energy is defined according to equation 2.1.4 within the Technical Manual. Essentially, it’s the energy received by the collector minus losses from the collector to the surroundings.

So in summary, you should see non-zero pump power when “Q useful” and “Irradiance – Incident” are both non-zero. Otherwise, you should see zero pump power. The rationale behind this, is that the solar water heating pump cycle will not run to save energy while the resource is not available.

Regards,
Nick

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