The monitored performance of a prototype STACS system mounted on a rooftop in Cardiff, Wales showed that a thermal COP of around 0.6 could be achieved in cooling mode, and that the efficiency of the evacuated tube collectors ranged between 0.7 and 0.85 depending on orientation and angle [7]. This data was used in the modelling predictions shown above, so the STACS system and component efficiencies are based on performance actually achieved using Wales weather conditions.

3. Carbon Emissions from the Welsh Housing stock

Using Table 2 and assuming all the current heating and DHW thermal demands are met from gas, and the cooling demand is met from electricity, then we can convert these demands into Carbon Dioxide equivalent emissions using the latest UK conversion factors [11]. The DEFRA guidance suggests using 0.43 kgCO2/kWh emissions factor for electricity for displaced electricity, and 0.185 kgCO2/kWh emissions factor for natural gas (gross CV). Table 3 shows the calculated annual carbon dioxide emissions figures per house type using these figures. It can be observed that the predicted annual carbon dioxide emissions are between 40 and 60 kgCO2/kWh per m2 regardless of the age or size of property.

No	Dwelling type	Elec Use (kgCO2) (kgCO2/m2)	Space Heating (kgCO2) (kgCO2/m2)	Space Cooling (kgCO2) (kgCO2/m2)	DHW Use (kgCO2) (kgCO2/m2)	Total Emissions (kgCO2) (kgCO2/m2)
1	Pre-1850 Detached House	1,329	15.2	2,061	23.6	12.9	0.1	401	4.6	3,804	43.6
2	Pre-1850 Converted Flat	1,329	12.8	4,606	44.5	38.3	0.4	401	3.9	6,374	61.6
3	1850-1919 Semi Detached House	1,329	6.0	8,954	40.7	10.3	0.0	401	1.8	10,695	48.6
4	1920-1944 Semi Detached House	1,329	14.2	3,673	39.4	24.1	0.3	401	4.3	5,427	58.2
5	1945 — 1964 Low-rise Flat	1,329	20.2	1,660	25.3	57.2	0.9	401	6.1	3,448	52.4
6	1945-1964 Semi-detached House	1,329	14.9	2,617	29.3	126.0	1.4	401	4.5	4,473	50.2
7	1965-1980 Detached House	1,329	11.4	3,268	28.0	42.6	0.4	401	3.4	5,041	43.2
8	1965-1980 Mid-terrace House	1,329	12.6	3,088	29.3	29.7	0.3	401	3.8	4,848	46.0
9	1981-1999 Low-rise Flat	1,329	29.7	673	15.0	225.8	5.1	401	9.0	2,629	58.8
10	1981-1999 Mid-terrace House	1,329	23.8	1,101	19.7	53.3	1.0	401	7.2	2,884	51.7
11	2000-2006 Semi-detached House	1,329	17.7	1,969	26.3	10.8	0.1	401	5.4	3,710	49.5
12	Post-2006 High-rise Flat	1,329	23.1	443	7.7	100.6	1.8	401	7.0	2,273	39.6

Table 3. Predicted annual carbon emissions for electricity, space heating, cooling and domestic hot water

Figure 3, derived from the data used in Ampatzi [5], shows the predicted solar yields and direct solar use in each dwelling as a percentage of the predicted dwelling annual thermal demands. It can be seen that the dwellings are capable of generating between 20 — 95% of their annual thermal demands via their collectors, but can generally only displace between 10 — 20% of these thermal demands from use of the solar energy when little or no storage is present.

Table 4, again derived from the data used in Ampatzi [5], shows the predicted carbon emission savings that could be made in the modelled dwellings through use of the tested solar thermal system and components as discussed in Agyenim [7]. Two sets of savings are shown — the first is with the direct use of Solar Thermal within the dwelling, i. e. with little or no Thermal Energy Storage (TES). The second set of savings show the potential savings achievable if sufficient TES capacity were installed to use the currently excess solar yield in each house. An overall exergy efficiency for the TES system of 40% is assumed. A further set of calculations still need to be undertaken showing the TES capacity needed to achieve various solar fractions in the houses with realistic system losses. However these two examples help to set reasonable limits to the range of carbon emission savings practically achievable within each house type.

Table 4 shows that, considering only the direct use of Solar Thermal within the dwellings, the potential reduction in the dwellings’ total carbon emissions (i. e. including electricity) range from around 4% to 11%, with 8 — 10% savings being achieved in most of the buildings. The table also shows that the use of a 40% efficient TES system could increase these savings to between 6% and 25%. As a general

observation it appears that TES could potentially double the potential carbon savings, though this would clearly depend on the design of the TES system.

The table also shows that thermal demands only constitute around 70% of the total carbon emissions from domestic properties in Wales. If we consider only these demands then Solar Thermal can potentially save around 14 — 36% of these carbon emissions.

Table 4. Predicted reductions in total carbon emissions through the direct and indirect use of solar thermal energy to meet the thermal demands in each dwelling. No 1 Dwelling type Total (kgCO2) Emissions thermal demands only (kgCO2) Direct fraction of total thermal demand Solar carbon emission savings (kgCO2) % reduction in carbon emissions from direct solar use Solar Yield fraction of total thermal demand Carbon emission savings — 40% efficient TES (kgCO2) % reduction in total carbon emissions using TES Pre-1850 Detached House 3,804 2,475 0.12 303 8% 0.40 576 15% 2 Pre-1850 Converted Flat 6,374 5,045 0.13 657 10% 0.60 1595 25% 3 1850-1919 Semi Detached House 10,695 9,366 0.12 1,140 11% 0.48 2470 23% 4 1920-1944 Semi Detached House 5,427 4,099 0.12 509 9% 0.34 856 16% 5 1945 — 1964 Low-rise Flat 3,448 2,119 0.10 203 6% 0.20 292 8% 6 1945-1964 Semi-detached House 4,473 3,144 0.12 373 8% 0.39 711 16% 7 1965-1980 Detached House 5,041 3,712 0.11 398 8% 0.42 862 17% 8 1965-1980 Mid-terrace House 4,848 3,519 0.11 374 8% 0.41 801 17% 9 1981-1999 Low-rise Flat 2,629 1,300 0.22 290 11% 0.95 666 25% 10 1981-1999 Mid-terrace House 2,884 1,555 0.17 257 9% 0.72 602 21% 11 2000-2006 Semi-detached House 3,710 2,382 0.12 296 8% 0.33 494 13% 12 Post-2006 High-rise Flat 2,273 945 0.09 82 4% 0.20 125 6%

4. Conclusions

Whilst this paper cannot provide estimates for the potential savings from the housing types not assessed in this study, the similar levels of potential percentage savings predicted for each of the house types modelled indicates that these savings levels are likely to be repeated in other house types as well.

It would seem therefore that Solar Thermal systems could potentially contribute a reduction of between 10 — 25% in the total carbon emissions from the Welsh Housing stock using only the practically useful roof areas found on each dwelling. These levels of predicted Carbon Emissions savings are significant in the context of the overall Welsh Housing stock, including existing housing, assuming that the Case Study models are a representative sample of the full stock.

The findings from House 1 also imply that even old housing stock can be brought back into energy efficient use when current energy efficiency techniques are used in its renovation (house 1 being a listed building). In conjunction with Solar Thermal techniques these savings can be very substantial.

Overall, it would appear that Solar Thermal should be strongly considered as an ingredient in Wales’ move towards a low carbon economy.

Acknowledgements

This research is supported by a grant from the Wales Energy Research Centre.

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