Water and Some Wacky Graphs

Richard Does Some Sums

Just for fun I did some calculations on stores and usage for rainwater harvesting  at Glebe House.  The conclusions are:

1.  The mean rainfall at Wallingford is 591 mm per year (1961-2011). Given a roof area of 75 sq m this gives 120 mm per day on average – but of course rainfall isn’t evenly distributed through the year.

2.  I did a calculation using the Wallingford daily rainfall (1961-2012) using a simple tank model.  Here are some graphs for 2000-2012.

 

The conclusions are:

a.  with a water usage of 200 L per day (ie 2 people using 100 L per day) the tank is empty between 50 and 60% of the time.  It doesn’t make a huge difference as to the size of tank (I tested between 2000 and 10000 L – the 10000 L tank just doesn’t fill up) – this is sort of obvious given the roof can on average only supply 120 mm per day.

b. if we reduce the water demand to 100 mm day then a 5000 L tank can supply water 94% of time. If we reduce the size of tank to 2000 L then the system can only supply water 78% of the time.

My conclusion is that even a 2000 L tank would be significantly useful but we probably need something of the order of 5000 L.

 

 

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Where does all that heat go?

Tim’s people came up with a full Passivhaus assessment of heat losses from Glebe House. Richard analysed this, along with the heat loss data he had taken from the dataloggers that measured inside and outside temperatures.

This is what we found:

PassivHaus Assessment

This analysis of existing energy loss, started with a ‘blower’ test to quantify heat losses through leaks and to identify major sources of leaks, followed by an assessment of possible improvements.

The house needs 21000 kWh a year to heat it to 20 degrees (Table 1).

Table 1. Heat losses from existing house by PassivHaus assessment

Heat loss/gain (loss positive) Value, kWh a-1
Transmission (floor, wall, windows, doors, roof) 21840
Ventilation heat Loss (draughts/leaks) 2890
Solar gain -1734
Internal heat gain (assuming 3 occupants) -1832
Annual heat demand 21165

The major heat losses are through ‘transmission’ (see Table 2).

Table 2. Heat loss calculation from individual components

Building element Area (m2) U value (Wm-2K-1) Temp. factora Annual heat loss
(kWha-1)
% of total transmission losses
Exterior walls 199 0.612 1 7982 37
Roof/ceiling 117 0.357 1 2729 12
Floor 89 0.739 0.46 1988 9
Windows 39.4 3.46 1 8919 41
Exterior door 1.9 1.8 1 223 1
Total       21840  

aAllows for different heat-losses to ground compared to air

Major losses are from the windows (8919 kWh) and the exterior walls (7982 kWh).

Green Factory analysed possible renovation options (Table 3).

Most cost effective options are  external cladding, which would cost, up-front, £1.20 for each kWh that would then be saved every year, and the windows, £2.20 per kWh saved every year (Table 3).  Upgrading the windows would also be likely to halve ventilation losses.  Upgrading the roof insulation, and replacing the front door, were not costed,  but are very likely to be cost-effective (and comparatively affordable).

It is debatable whether insulating the floor slab is worth the considerable cost (between £9 and £14 per kWh saved every year).

Full renovation would save 13217 kWh per year, equivalent to 2683 kg CO2 per year (assuming the energy is supplied by natural gas with 0.203 kg CO2 per kWh).

Table 3a.  Renovation Option 1

 

Area

Current

Option 1a

m2

U
W/m2k

Heat loss
kWh/a

U
W/m2k

Heat loss kWh/a

Cost
£

Cost per kwh/a saved
£

 

199

0.61

8000

0.17

2200

7000

1.21

Roof

117

0.36

2700

0.11

800

Floor

89

0.74

2000

0.28

800

11400

9.25

Windows

39

3.46

8900

1.80

4600

9900

2.30

Door out

2

1.80

200

1.40

200

TOTAL

21800

8600

aOption 1: : 100mm wall insulation, double glazed windows, sempafloor

Table 3b.  Renovation Option 2

 

Area

Current

Option 2b

m2

U
W/m2k

Heat loss
kWh/a

U
W/m2k

Heat loss kWh/a

Cost
(£)

Cost per kwh/a saved
£

 

199

0.61

8000

0.17

2200

7000

1.21

Roof

117

0.36

2700

0.11

800

Floor

89

0.74

2000

0.24

600

19400

14.48

Windows

39

3.46

8900

0.80

2100

13800

2.01

Door out

2

1.80

200

1.40

200

TOTAL

21800

5900

bOption 2: 100mm wall insulation, triple glazing, 75mm kingspan floor

Dataloggers gave us limited information, as they were not in for long: the heating keeps the hall at a fairly constant 19OC; the bedroom is 3 to 4 OC cooler than the hall  – consistent with the thermal image and probably due to inadequate radiators in this room; and the interior wall of the house is a degree or so cooler than the air – consistent with a substantial loss of heat through the walls.

Energy consumption

From early March, we monitored electricity use automatically with an ‘OWL’ wireless electricity monitor, gas use via daily manual readings, and daily generation of the PV array (manually monitored).

The major use of energy is gas (average 49 kWh per day compared with 8 kWh for electricity).  We use gas for background heating and hot water, and electricity for lighting, cooking and electrical appliances.  During this amazingly sunny period, PV solar generation was 10 kWh per day, 20% higher than the electrical consumption.

The measured energy use is consistent with EPC and PassivHaus analyses. It broadly followed the external air temperature, although the series is too short to develop a quantifiable relationship.

Read more about the analysis, in the full report: Demo Glebe house

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Door-blower test

On January 13th 2012 we won a grant from the Department of Energy and Climate Change to monitor Glebe House and assess the heat losses prior to eco-renovation.

We set up ‘dataloggers’, to take the temperatures inside and outside the house, the sensors sellotaped to the wall to get the wall-heat or 3 inches out into the air to get the air-heat.  The cooling effect of the single-glazed windows upstairs are now complemented by a whistling breeze through the bore-holes in the window-frames that take the wires through from inside to outside.

In addition, there is an electricity monitor giving us constant feedback on how much electricity we are using. You should all get one of these! It makes you much more conscious of how much water you put in that 3kW kettle.

Tim Fenn of the Green Factory is a designer of ‘green’ buildings and he brought in his own monitoring devices. Paul and Jazz turned up first of all with the ‘door-blower’ test.

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Money money money

Quotes came in from the builders: to convert the roof, change the garage to a workshop and utility room, upgrade the bathroom and put in a second bathroom, put in underfloor heating, and to externally clad the house all the way round and put in triple glazed windows.

Quotes were 50% to 100% higher than what we could afford and what we anticipated it would cost.

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Greening the garden

Ain’t nobody here but us chickens

And partridges (round the pear tree)

And finally the Greenhouse arrives

Before

And after

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Solar panels get the FITs

Disaster!

The Government suddenly pulled the plug on the feed-in-tariffs for solar photovoltaics. The rate was to be slashed to half from a start date before the consultation had even ended! We rushed out to buy our panels before the 12th December 2011.

This was a fraught period – no scaffolders to be had for love nor money, the supplier ran out of invertors, and then out of aluminium runners to hang the panels off. But our chaps came good in the end. NRG 8 installed almost 4 kWp of panels on frames in the garden and the panels have been doing their stuff ever since.

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First-Pass Plans

Summer fun

Richard and I married in July 2011 and were finally both living at Glebe House. Wedding gifts were plants for the garden.

Dream on

We set about planning what we wanted to do to both house and garden. Richard’s priority was a greenhouse, mine a converted roof room to look out over the Chiltern Hills.

And we both wanted a super-insulated, super-eco-renovated house with external cladding, triple-glazed windows and the most exotic array of ground-sourced heat pump, well, rainwater harvesting, photovoltaics, solar thermal, log-burning stoves, underfloor heating – the works.

We slowly got plans drawn up and planning permission obtained. We wanted to re-model the front swathe of tarmac and develop a garden with swimming pond. The cars would be corralled into a place of their own, with a south-facing carport  to take a decent array of solar photovoltaics.

 

 

 

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