Atrium
The atrium is an entirely passive device with no emissions associated
with its operation. There will be an energy cost associated with
materials and construction, but as the atrium is constructed of
conventional materials and as the building needed an enclosed stairwell
anyway, the net environmental penalty of building the atrium is
negligible.
Heat pump
The heat pump system, as a consumer of externally supplied energy, has
significant environmental impact. We need to assess this impact in
comparison to that of the alternatives. There are three main areas of
concern: emissions, location of pollution, and the sustainability of
the groundwater heat resource.
Emissions
The combustion of fuels can cause emissions of a range of pollutants.
Carbon dioxide is produced when organic fuels are burned. Where combustion is incomplete (for example, when the air supply is restricted) there may be emissions of carbon monoxide, unburned hydrocarbons or particulates (e.g. smoke). The use of any sulphur-bearing fuel results in the emission of oxides of sulphur, and any high-temperature combustion process is likely to produce oxides of nitrogen. Between them, these pollutants have environmental effects, which can be local, transnational, or global; examples are smog, acid rain, and global warming respectively.
The nuclear generation of electricity can be regarded as producing emissions in the form of radioactive waste, the permanent and safe disposal of which is still an unsolved problem.
We considered the output of three major gaseous pollutants: carbon dioxide, oxides of nitrogen (NOx) and oxides of sulphur (SOx). We used the Emiscal program (REFERENCE) to estimate the emissions of these three pollutants for four different systems: an electrically driven heat pump, an engine-driven heat pump, a gas boiler, and electrical resistance heating.
We assumed that electricity was generated by the current UK plant mix, that the heat pump was powered by a natural-gas-fueled internal combustion engine, and that the gas boiler was a modern condensing boiler with average efficiency of 80%. All other parameters were the same as those used in comparing the energetic efficiency of these systems. We estimated the emissions associated with the delivery of 1 kWh of energy in the form of hot water at 60?C.
The results for carbon dioxide are shown in Figure 1. The CO2 emissions from the engine-driven heat pump are lower by a factor of about 2 than those from the electrically-drive heat pump: this we would expect from the lower overall energy use. The SOx and NOx emissions are shown in Figure 2. The sulphur emissions of the engine-driven heat pump are much lower than either of the electric alternatives. The NOx emissions of the engine-driven heat pump are comparable to those of the electric systems.
These figures will vary a lot with circumstances. For example, it may be necessary to use oil to fuel the engine, and the rise in renewables and the increasing application of flue-gas desulphurisation will reduce the SOx emissions of the electric options. Both of these factors would make the SOx comparison less flattering to the engine-driven heat pump. In contrast, we may be able to use an external combustion engine such as a Stirling engine, whose lower peak combustion temperature will result in reduced NOx.
Location of pollution
Although the engine-driven heat pump may reduce emissions compared to the electric alternatives, it will cause them to be emitted in
populated areas, where their effects may be more intense. In addition,
it may be easier to provide pollution-control equipment for a few large
power plants than for many small ones. Choosing a suitable trade-off
between these factors depends upon one’s priorities, and here we simply bring attention to the issue without suggesting a solution.
Sustainability of groundwater heat resource
At present we have no information about either the mass flow rate of the groundwater source at Shettleston, the ultimate destination of the
water, or of the rate of heat transfer into it. We are therefore
unable to assess whether the continuous abstraction of 60kW of so will
have significant environmental impact or whether it can be continued
indefinitely without cooling the source to the point where the
coefficient of performance of the heat pump is affected. Even if it
can, we have to consider what might happen if many people decided to
extract heat from this source.
Photovoltaics
Photovoltaics provide an energy payback well within the life of the
system and are therefore likely to provide a benefit in terms of
emissions. Zurutuza and Bastarrika (personal communication) estimate that in a Scottish climate, the life-cycle CO2
emissions of a photovoltaic system are equivalent to about 0.2 kg
kWh-1. This is approximately one-quarter of the CO2 emissions from the
current UK plant mix. In operatIn operation, photovoltaics are silent,
unobtrusive, and emission-free.