Water heater consumption (kWh)

asked 2019-02-22 13:59:14 -0500

updated 2019-02-22 19:52:16 -0500

Hi, I'm trying to simulate a stratified water heater for São Paulo-Brazil .epw and find out the consumption of the heater (kWh) for each month. I'm using a heater of 6000W capacity, with a patter of bath 40minutes per day (4l/min). I got the consumption from the Table Report and it is too high (The usual is maximum of 90kWh for a month). I converted J to kWh and got this:

WATER HEATER TOTAL DEMAND HEAT TRANSFER ENERGY [J](kWh) WATER HEATER HEAT LOSS ENERGY [J]   (kWh)
January          482239000.00                                     133.96    -79534413.60                                           -22.09
February             443646000.00                                     123.24    -73541655.60                                           -20.43
March            538367000.00                                     149.55    -85841829.00                                           -23.84
April                    550458000.00                                     152.91    -94792808.10                                           -26.33
May                  600509000.00                                     166.81    -105807662.30                                          -29.39
June                     580653000.00                                     161.29    -102193879.20                                          -28.39
July                     612884000.00                                     170.25    -108955406.70                                          -30.27
August           591676000.00                                       164.35  -103599575.70                                          -28.78
September    556954000.00                                     154.71    -96280359.00                                           -26.74
October          578996000.00                                     160.83    -99992914.30                                           -27.78
November             543994000.00                                     151.11    -93088050.10                                           -25.86
December             540370000.00                                     150.10    -89565130.60                                           -24.88

The temperature of the tank outlet looks fine. I dont know if I'm using the wrong output object to obtain the consumption results or if there is a mistake in my .idf. I'm senting a link if somebody want to give me a little help here.

https://www.dropbox.com/sh/or7f0yk7p6... Thanks!

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answered 2019-02-22 18:10:07 -0500

rraustad

13599 ●72 ●12 http://www.fsec.ucf.edu/

updated 2019-02-22 19:49:06 -0500

You have what appears to be a large loss coefficient.

WaterHeater:Stratified,
    Storage Tank Stratified, !- Name
    0.15,                    !- Node 1 Additional Loss Coefficient {W/K}
    0.1;                     !- Node 6 Additional Loss Coefficient {W/K}

10% to 15% in losses seems high to me but I am not an expert in water tanks. Try decreasing these losses and see if the tank acts more like you would expect (e.g.,divide losses by 10 to 0.015 or 0.01 W/K)

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Hi! I tried that but it didn't change much. The demand is still too high for month (january that is summer, for exemplo, 112kWh).

VanessaCosta ( 2019-02-22 19:17:44 -0500 )edit

Can you clean up that table in your description so I can see it better? The losses are about an order of magnitude lower than the heat transfer (so heat transfer should be predominant). I expect that you just don't understand the results. Let's narrow this down.

Set the losses to 0 until you figure this out.
Compare the water draw schedule (profile) to the heat transfer (profile) to see if they match up

rraustad ( 2019-02-22 19:23:47 -0500 )edit

WATER HEATER TOTAL DEMAND HEAT TRANSFER ENERGY [W] January 130898.61 February 120408.61 March 146261.11 April 149282.78 May 162756.11 June 157383.06 July 166070.83 August 160386.94 September 151023.61 October 157001.39 November 147546.67 December 146673.90

VanessaCosta ( 2019-02-22 19:55:03 -0500 )edit

WATER HEATER HEAT LOSS ENERGY [W]
January -19.04 February -17.60 March -20.55 April -22.69 May -25.32 June -24.46 July -26.08 August -24.80 September -23.04 October -23.93 November -22.28 December -21.44

VanessaCosta ( 2019-02-22 20:01:41 -0500 )edit

Something is wrong. You have a 6 kW water heater with a 130898 W demand for January. We have to figure out what that means before proceeding. Is that W or kWh?

rraustad ( 2019-02-22 20:03:16 -0500 )edit

Good, you have nearly zero'd the losses. Why is not 0?

rraustad ( 2019-02-22 20:04:27 -0500 )edit

Once the hot water pattern is constant (between 18:00 and 18:40 p.m.) 0,16m³ per day

VanessaCosta ( 2019-02-22 20:04:36 -0500 )edit

For January is 130898Wh, I supposed

VanessaCosta ( 2019-02-22 20:06:37 -0500 )edit

once the results are given per hour in Joules (J) i changed it for kWh

VanessaCosta ( 2019-02-22 20:07:40 -0500 )edit

You're right, I will set the losses to zero.

VanessaCosta ( 2019-02-22 20:10:14 -0500 )edit

I was comparing the 130 kWh to my own residential HW energy use (I've measured). About 3 kWh/day or 90 ish kWh/month. That's for 2 people.

rraustad ( 2019-02-22 20:30:06 -0500 )edit

I got the losses close to zero. But the sum of the demand of electricity for the element heater is still high: January 111.86kWh February 102.80kWh March 125.87kWh April 126.77kWh May 137.63kWh June 133.11kWh July 140.21kWh August 135.79kWh September 128.14kWh October 133.24kWh November 125.44kWh December 125.39kWh

VanessaCosta ( 2019-02-22 20:31:01 -0500 )edit

Well, i make sense. Water temperature here is abou 20°C at summer and 15°C at winter

VanessaCosta ( 2019-02-22 20:34:34 -0500 )edit

You have 111 kWh's for Jan (domestic HW only I assume). I have measured my house and see about 3 kWh/day on water heating which is 3 kWh/day * 30 day = 90 kWh/month . You are very close.

rraustad ( 2019-02-22 20:51:55 -0500 )edit

Thanks a lot, Richard!

VanessaCosta ( 2019-02-22 21:04:14 -0500 )edit

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