Wednesday, May 19, 2010

House-Heatpump Electricity Table

19 May : More about the long call with David Atkins. Over the coming year, with good records being maintained, we will have to chance to see actual electricity consumption compared with theoretical. The figures we have already are very encouraging.
Let's look at the house and heatpump more closely.

Last night, I came up with a figure of approx 14,500 kWh for the year for our house, based on some very rule of thumb calcs for UValues and Airchanges, floor space, volume and average fenestration.
    He ran the details of our house through the estimating web page provided by IVT (VPW2100) and I hope in future that I can have an access ID to use the same. I have found similar sites with 'VPW2100', eg the Bosch site, but it is entirely in Danish and much of it in Flash, so google can only translate the first page.
   For our house size and occupancy and location, David's software came up with an annual heating demand of 14,590 kWh. I am quite pleased at how close that is to my first estimate.
   14,590 kWh can be approximately divided into 11,600 kWh for space heating and 2990 kWh for domestic hot water (based on a predicted percentage split). That is still a long way lower than the average British gas heated medium size house of about 25,000 kWh.
  Of this 14,590 kWh, the predicted heatpump overhead is likely to be 4,800 kWh for pumping, compressor etc, and the expended heat goes into the house. Therefore, the amount extracted from the ground is expected to be 9790 kWh. The VPW page prints out a Histogram with monthly estimates. The table below starts in September 2009 when I put a meter onto the GSHP, and compares the predicted workload and performance for the GSHP compared with the actual. If these figures are to be believed, it seems that a very pessimistic expectation is made for the COP. We also have an extra overhead that during the winter, the underfloor heating system pump burns 125W all the time, also working through the GSHP's meter. This pump adds 60 kWh per month to the monthly figures, which makes our March and April figures look even better, as the theoretical figures predict the GSHP performance without the auxiliary pump.
We shall see after a year how all this pans out. Here's the table.

Monthly 'Expected' GSHP and PV Roof performance, against 'Actual',
balanced by contribution from Sunboxes. PV harvest indicates 'sunniness'.
Month  GSHP expect 
consume kWh
kWh expect
from Ground 
GSHP 
kWh
GSHP
w/o aux
Sunbox 
kWh
PV Roof PV
Expect
Comment
 Oct 09 259 530 221 190 170.6 166
 Nov 09 580 1284 405 345
109.3 81
 Dec 09 721 1548 681 621
64.4 49
 Jan 10 820 1730 797 737
57.3 70 Snow
 Feb 10 742 1558 750 690
98.6
122 Cold
 Mar 10 652 1423 513 453 227 281.3 217 SB start 7 Mar
 Apr 10 424 910 213 193 321 428.7  323 Sunny
 May 10 156 254 155 135 280 491.0 421 cold-sun-cold
 Jun 10 110 139 71 71 375 513.0 407 HW only
 Jul 10 110 139 45 45 315 432.6 427 HW only
 Aug 10 110 139 66 66 302 382.8 362 HW only
 Sept 10 110 139 100 90 280 278 263 HW + Heat
Our heatpump workload is generally lower than expected because we turn it off overnight, due to the good insulation of the house, and the wish to reduce floor circulating noise. The figure was higher in Feb due to the colder winter, and the GSHP sensors misbehaving and over using additional heat for some days. That has been fixed.

It is too early to be sure until more months elapse, but at first glance, the reduction in workload for the GSHP in April is quite stunning, it is less than half the expected amount. April was a sunny month. Then the first 2 weeks of May were surprisingly wintry, and the May GSHP consumption finished up as 155 kWh, about the same as the VPW prediction, because of the very cold start (and end) to the month.
I shall return to this article later when the monthly figures roll in.

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