Tapping into water

The problem with justifying any design is that it is reliant upon suppositions about requirements and as the author Stuart Brand has noted “All is design is a prediction, all predictions are wrong.” Key amongst the assumptions those about the behaviour of the occupants. In terms of whole systems design these assumptions can be critical. This reason for this criticality is that it is the whole life savings that help to justify any additional capital cost, the larger the savings the more you can afford to expend on low carbon technologies.

Say for example I know of an efficiency measure that, with reasonable certainty, will deliver an 80% saving compared to the business as usual technology (BAU). If I were to assume that you use the appliance for say ten hours per day, or perhaps ten times per day then and 80% saving will deliver significant results, but what if in reality you only use the applicant for five hours, or five times, per day? You will still have an 80% saving compared to the BAU technology but the actual savings will fall by 50%, thus the whole life benefit is much less than it originally appeared. On this basis, when using whole systems analysis to justify investments, a great deal of caution has to be applied, or at least it has to be appreciated. Everyone involved in the process has to buy into it and agree the design parameters. Ideally agreement could be based upon statistics gathered from in-use behavioural surveys that allow accurate predictions to take place. Sadly such information is not readily available.

For a long time I have had an interest in the potential to reduce water consumption and the use of domestic hot water. The key issues here is how far can you go without compromising comfort and service? Through out the study I do not anticipate or require any behaviour change by the building occupants; this is important as whilst behaviour change is beneficial and can offer further value engineering it cannot be ensured and thus, unless you have very willing occupants, cannot reasonably be not be relied upon.

Another point that I would like to make is that thought the use frequencies are based upon reference material from reasonably reputable sources (such as the Environment Agency) I do not know how these use frequencies were established and thus can not be certain of their appropriateness.

So with these words of warning I would like to look into the heady topic of water conservation and how this may be used, through whole systems analysis, to low carbon technologies such as solar hot water.

For expediency I will not discuss each and every technology and its savings potential. The table below shows the BAU technologies and contrasts them with the Best Available Technologies (BAT). It also indicates the savings per use, thus not reliant upon behaviour, and the assumed frequencies of use per day that are used to assess the affordable cost of solar hot water.

NOTE: Losses have been assumed to be reduced to zero through a leak detection system and Car Washing + Garden Use has been reduced to zero through the use of rainwater harvesting using a water butt.

SHW System Design

The typical SHW system of course does not assume that you have introduced water conservation technologies as a consequence the design of the SHW system could be optimised so as to increase solar fraction, reduce capital costs and foreshorten payback period even further.

Amory Lovins, a world-leading expert in energy efficiency, reports that efficient end-use, achieved by using water conservation measures, is reported to normally entail roughly 0.3-0.7m3 of storage per m2 of collector.

Water Distribution

Other savings can be made by using micro-bore pipes rather than traditional copper pipes and with good design you can reduce the pipe runs. Both of these considerations can help to reduce distribution losses.

Cost Benefits of Water Efficiency

Running through the calculations it turns out that across the household you can save about 55% of the water without any discomfort and inconvenience of little or no capital cost (the calcs suggest that this saves about £55/yr per person). To keep this study simple I have assumed that with good shopping around you can find all the BAT technologies at zero marginal cost (at worst a payback of one year). An added benefit is that with these self same efficiency measures is that you can also reduce your hot water use, thus saving on your energy bills also here you can save about 51% your domestic hot water (the calcs suggest that this saves about £30/yr per person). By adding the value of these two savings together you save £85/yr per person, thus a four person house saves about £340/yr.

In the calculations I have assumed that a condensing gas boiler has been used; thus the savings exclude benefits that may be accrued through insulating the hot water tank (and the supply pipes.)

Cost Benefits of SHW

The cost of a solar hot water system is about £2000-2500 per home for a typical installation (I’ll assume a worst case of £2500). Thus by using the accrued savings from energy efficiency payback could be achieved in 7.35 years relying upon the conservation measures alone. Given a fairly typical SHW system has a lifetime of 25-30 years the cost benefits are considerable (I’ll assume a worst case of 25yrs). If you then factor in the energy benefit from the solar hot water the picture gets even rosier….

One manufacture reports that in a study period 2006-2007 their system achieved an Annual Solar Energy Input of 1,448 kWh. Every day the tank was heated to 60C whether or not there was sufficient solar heat. Over the 30year life expectancy the system would achieve a Lifetime Solar Energy Input of ~43,230 kWh. (This manufacture reports a cost of £1000 for the system, to ensure a worst-case assessment in my calcs I’ll stick to the £2500 figure.) So

£2500 / 43,230 kWh = £0.057 /kWh or 5.7p /kWh

Now this rightly appears to be expensive but we have not considered the value of the water efficiency savings (both mains water and DHW). My calcs based upon utilisation frequencies suggest that it is possible to achieve a saving of £85/yr per person. Therefore:
DHW + Water = £85.37
kWh used = 870 kWh
Therefore the Permissible additional energy cost:
£85.37 / 870 kWh = £0.098 /kWh

and the Affordable energy cost = Permissible additional energy cost + existing energy cost:
£0.098 /kWh + £0.035 = £0.133 /kWh or 13.3p/kWh

Based upon a given set of design assumptions this calc indicates that we can afford to pay up to 13.3p/kWh for our SHW without fear of incurring additional whole life costs.

The Frailty of Whole Systems Analysis

As discussed earlier the value of Whole Systems Analysis hinges on making the correct assumptions. Using the frequency analysis derived from studies by the Environment Agency and a report to the Scottish Executive the above conclusions are valid however, the English House Condition Survey (EHCS) estimates that households use around 20-25% more hot water than is currently being applied in the fuel poverty modeling using BREDEM and survey data indicates that consumption is in the order of 49 litres of hot water per person per day.

In contrast my calcs using frequency data suggest that DHW consumption would reduce from 70 litres per person to about 40 litres/person per day using water efficiency. It is concluded that the frequency data would produces inaccurate results for hot water consumption. As we don’t have accurate data sets that agree with one another things begin to get confusing. To make matters worse the average European household, according to EU m324EN, reportedly consumes the equivalent of 100 litres at 60C throughout the day (I presume that this is a 3 person household giving 33lts/person). Which results do we rely upon and how do we estimate the savings in DHW as generated by water efficiency?

Lets stick to UK data so that we have greater appreciation of the UKs culture of DHW consumption. But in order to make this process more challenging lets assume that we make 40% saving (from 49lts to 29.4lts) rather than the 40% saving (70lts to 40lts.)

Without further data we’ll have to assume the mains water consumption is unaffected i.e. remains at £55.82 /person per yr but now DWH savings fall to £14.4 from £24.0 saving £9.6 (rather than £29.5.)

The annual saving is now
DHW + Water = £65.42 /person per yr
kWh saved = 275 kWh
kWh used = 411 kWh

Therefore the Permissible additional cost of energy =
£65.42 / 411kWh = £0.159 /kWh

and the Affordable energy cost = Permissible additional energy cost + existing energy cost:
£0.159 /kWh + £0.035 = £0.194 /kWh or 19.4p/kWh

I’m sure that you’ll agree that this change is significant and furthermore you may note that this is perhaps not the result that you’d have expected. So why does is appear that we can spend more money per kWh than we conserve less energy? Well, this is because so much of the value of saved capital is associated with the mains water savings and not the DHW. Whilst the energy savings have reduced the value of savings has remained disproportionately high, as a consequence we can afford to spend more money to displace on unit of energy.

So, if we exclude the value of the water conservation and focus only on the energy savings derived from water conservation then we can get a feel for how reliant we upon mains water to cover any short fall. The value of the energy savings alone permits a SHW cost of 5.8p /kWh where the:

Permissible additional cost of energy =
£9.6 / 411kWh = £0.023 / kWh
and the Affordable energy cost = Permissible additional energy cost + existing energy cost:
£0.23 /kWh + £0.035 = £0.058 /kWh or 5.8p/kWh

As the cost of energy conservation is roughly equal to that of SHW we could consider the whole system to be cost neutral even at the exclusion of the capital value associated with mains water conservation.

Cost/kWh Saved

The traditional energy efficiency focuses upon saved energy i.e. the cost/kWh saved (or negawatts as some would say). Under this scenario the cost/kWh saved of SHW with water conservation measure can be calculated:

Whole life = 25 years
Value of saving: £78.14 / person
KWh consumed by BAU: 1713 x 30yrs = 51,390 kWh / person
KWh consumed by BAT: 870 x 30yrs = 26,100 kWh / person
kWh saved: 844 x 30 years = 25,320 kWh / person

Lts consumed by BAU: 230 x 30yrs = 6,900 lts / person
Lts consumed by BAT: 113 x 30yrs = 3,390 lts / person
Lts saved: 117 x 30 years = 3,510 lts / person

Number of occupants = 4 people
Utilities at:
Gas = £0.035 /kWh
Water = £0.0013 /lt

Therefore cost/kWh saved (including saved capital expenditure on SHW and Water Conservation) =

((marginal cost) + (whole life cost of BAU – whole life cost of BAT)) / energy saved = cost / kWh conserved

£2500 - ((£0 + £16,941) - £8,405)) / 21,103 kW.h = £ -0.286/kW.h

Thus, under these design conditions, when considering the whole life cost of system, the cost of investment to displace 1kWh of hot water is negative i.e. it is a cost saving. As the cost /kWh of the system is less, significantly less, than that of gas the whole system can be considered very economic at today’s prices.

Conclusion

Due to the apparent inaccuracy of the available data sets it is not possible to make a conclusive statement about the value of energy and water savings resulting from water conservation measures, however, based upon the range of analysis described above it is possible to state that, there are cost benefits and that these are very likely to cover any marginal costs associates with SHW.

Even under the most onerous conditions, when you exclude the value of the water savings and rely solely upon the energy benefit of water conservation measures, then the whole systems design of water conservation and solar hot water would appear to fall within a reasonable economic threshold.

If one factors in water consumption, and chooses to agree with the stated utilisation factors, then the combination of water conservation and SHW could payback in approximately 7.35 years. It can be concluded that from the range of view points discussed, even at today’s prices, the cost of SHW can be engineered to be a very economic proposal.


About the author:
Mark Siddall, principle at low energy architectural practice LEAP, is an architect and energy consultant specialising in low energy and PassivHaus design. He was project architect for the Racecourse Passivhaus scheme and has a keen interest building performance. In addition to architectural services his practice provides project enabling and education for clients, design teams and constructors.

LEAP website: www.leap4.it