Heatloss+Energy

See the Key Performance indicators page for a regular update of graphs of performance

Heatloss
This spreadsheet is an approximate calculation of the heatlosses of the Peveril Solar house: loss through Fabric, loss through Ventilation and loss from Hot Water heating.
    This isn't as detailed as a SAP (Standard Assessment Procedure), but a full SAP is difficult to do when the scenario is made so complex with a heatpump and underfloor heating, integrated water tank and sunboxes, evacuated tubes, heat exchanger and the rest.

This is using the more old fashioned method of just adding everything up, and multiplying out, using 'Degree Days' to arrive at an annual quantity!

Once you have insulated a house well, this calculation illustrates how strong is the argument for MVHR (Heat Reclaim unit) to recycle the heat in the air. It is an essential part of the Passivhaus concept.
    In this calculation, 4,295 kWh is through fabric losses, and 7,345 kWh through air replacement. I wish we had installed this during construction, as it so much more difficult to do later. MVHR is a waste of space and money, unless the house is constructed to be as airtight as a German house. Our house is not, but it is capable of being made more airtight. Every house is capable of being made more airtight. You can also apply Heat Reclaim to individual rooms or groups of rooms, and we have done so to our kitchen-dining suite.

Having worked out the heatloss, I then compared it with the electrical consumption for the previous 2 years, and came up with a very approximate Heatpump COP over 2 full years from Aug 07 to Aug 09 = 2.65, simply a ratio of the heat it is required to produce, compared with the electricity it actually consumed while doing it.
   Having talked with David Atkins of Ice Energy about heatloss calculation, he came up with a figure of approx 14,590 kWh for the year for our house, based on the estimating web page provided by IVT (VPW2100), He entered the approximate details above of the area, size, air-change rate and construction date of our house. This is amazingly close to the 14,548 of my calculation above, and produces roughly the same COP.

Performance
The annual heating load of 14,590 kWh can be approximately divided into 11,600 kWh for space heating (H-loss + vent loss) and 3,000 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 14,600 kWh, the predicted heat-pump electricity overhead is likely to be 4,800-5,200 kWh for pumping, compressor etc, and the expended heat goes into the house. Therefore, the amount extracted from the ground is expected to be 9,800 kWh. With our ascetic lifestyle, this is likely to be closer to 9,000 kWh. The VPW2100 page prints out a Histogram with monthly estimates. The tables below start 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. On the basis of these figures, a somewhat pessimistic expectation is made for the COP, of about 2.65 averaged over the year. This is what our Sunboxes have to improve on.

Below is the table for October 1 2010 to September 30 2011.

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 10
259
530
259
200
307
173.30
166
unusually cold
 Nov 10
580
1284
477
420
194
96.6
81
very cold
 Dec 10
721
1548
763
703
109
49.50
49
coldest EVER!
 Jan 11
820
1730
609
550
94
65.3
70
warm month
 Feb 11
742
1558
405
350
118
89
122
v cloudy, not cold 
 Mar 11
652
1423
354
320
298
264
217
v warm, sunny, dry
 Apr 11
424
910
102
96
346
416
323
v warm, sunny, dry
 May 11
156
254
67
66
299
502
421
warm month, cloudy
 Jun 11
110
139
58
58
337
548
407
warm month
 Jul 11
110
139
51
51
334
452
427
warm month
 Aug 11
110
139
52
52
322
376
362
average month
 Sept 11
110
139
47
46
322
314
263
sunnier month with 
v sunny end

Below is the table for October 1 2009 to September 30 2010.

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
Degree Days during this period: 2383
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. 
Because we have underfloor heating, we also have a auxiliary 125W water pump that drives the heating fluid through the narrow gauge pipes all day long whenever the air temperature externally is below 15º. This pump adds approx 60 kWh per month to the monthly GSHP electricity consumption figures, which makes our March and April figures look even better, as the theoretical figures predict the GSHP performance without the auxiliary pump. The column called 'GSHP kWh' is the meter reading and that called 'GSHP w/o aux' is my estimate of the GSHP consumption disregarding the pump overhead.

It is too early to be sure until more months or years elapse, but at first glance, the reductions in workload for the GSHP in April-August 2010 (since the Sunboxes worked) are quite stunning, it is fractions of the expected amounts. I now publish monthly summaries of weather and performance in the blog. Since the sun boxes were installed, the overall system shows a significant reductions in GSHP consumption. During the summer months, we need no heating at all (thanks to incidental gains and good insulation) and our small household has low hot water consumption. We had over 3,300 kWh of electric generation in each year that we have had the PV.
     For the house as a whole, combining the power-up contribution from the PV, and power-down savings from the Sunbox,  we have reduced annual electricity consumption from the regular 8,500 kWh to below 5,000 kWh.

      As explained above, about 9,000 kWh thermal will be drawn from the ground. The most that the 4 sqm of sunbox are able to put down is 3,000 kWh, about one third. Therefore, it seems that can be no expectation to raise the temperature of the soil, long term. Prevention of long term cooling is the primary hope - but is does mean that, in a sense, the sunboxes are 100% efficient - all the heat put down must come back up again due to the overwhelming ratio of Pull to Push. The loft pump consumption occurs mostly in the day during PV generating hours and is only 15 watts.

How does this compare with real weather?
This table shows records for the last three years, using Degree Days as means of recording the severity. If 2008 was typical, then 2009 was 5% milder, and 2010 has been more than 20% colder - and for those of us who lived through 2010, we agree! The diagram, below right, shows a comparison of the Degree Days (red) and GSHP workload (blue), and the frenetic activity of the winter 2009-2010 winter can be compared to the calm efficiency of the winter 2010-2011 (despite the severe autumn 2010).


When should there be reduction in the power consumption of the Heat Pump? 
I believe that there should be improvement at all times...
- During the Summer, there is so much warmth from the sunboxes that it has little need to get heat from the ground - the boxes provide it directly.
- Another time for improvement is during the Spring, when the ground has been giving up its heat for months, and there has been little sunshine, it has in previous years struggled, and switched over to 1:1 electricity burning at 6kW.
- During the Autumn, signs of winter are coming, with cold evenings, and the deep heat stored during the summer should easily deal with the demand. There is a diurnial contribution in the equinox times, with warm day heat stored and returned to the house in the evenings.
- We hope to see the heatpump finding it easier to get heat from the ground at all times of the year. During the first half of the winter, late Autumn, the GSHP  gets heat energy from wherever it is available, and there is plenty in the ground after a long summer.

The research team will continue to monitor performance of the GSHP and Sunboxes through Spring and Summer 2013, and onwards, and this will be the subject of later articles on inter-seasonal performance.

Calculations of electricity readings for the house are complex since Oct 2009 because of the daytime electrical contribution from the PV roof. The GSHP, PV and Sunboxes have their own meters, enabling us to know their consumption or generation, for sure.


Deep Ground Temperature
A key performance indicator must be the temperature of the ground. See the Key Performance indicators page. Since the Sunbox system was installed, the deep ground temperature has never fallen below 10.0ºC in three winters running (2010-11, 2011-12, 2012-13).

Is there a Payback aspect to Sunbox augmentation of a ground source heatpump? 
  Only a couple of winter's testing will prove that conclusively, in terms of reduced electricity consumption. My colleague Blaise Mempouo has pointed out that if the sunboxes are effective and permanent installations, then the size of the ground loop could be reduced - perhaps saving the entire cost of the sunboxes (although I regard a reduction in the length of boreholes as an unwise economy). The sunboxes are capable of repaying their entire cost by electricity savings - I can't say how short a time as there isnt a 'production cost' on them yet.
    Boreholes get more expensive when they are deeper, because of the size of rig required. The risks of encountering bad ground increase. With solar earth charging, you can consider the cheaper proposal of a cluster of shallow boreholes, still achieving the aggregate of metres of length, just in case the solar charging equipment is taken out by future owners (for example, five 20m deep holes instead of one 80m hole). Meanwhile -  just enjoy the improved GSHP performance - reduced electricity bills and faster heat up by the GSHP, greater longevity of the GSHP.


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