It is a political priority that our new buildings and our new city districts shall be sustainable. But how do we find the most sustainable energy solutions? We have an approach and an answer.
The general opinion is that a building is more sustainable if it is a low energy house or even better a zero energy house having solar cells and wind turbines on the roof and highly insulated walls. We come in general to the opposite conclusion.
We have analyzed the problem and came to a more differentiated answer.
There is a general agreement that you in order to evaluate the sustainability have to include the following 3 criteria:
the economical resources,
the environmental impact and
the social performance.
A sustainable energy solution for a building is a solution which in a long life-time will provide the users with good thermal comfort at the lowest life-cycle cost including all environmental costs and being able to be independent of fossil fuels in the long term. Besides it shall also have a positive impact on the social environment. In other words we shall develop our cities to create good conditions for our grand grand children.
So how do we compare two energy solutions with respect to sustainability? Please note that it is only the characteristics of the energy solutions – not solutions for water, waste water, materials, urban development etc.
In Denmark the ministries have developed a methodology for economical assessment at the national level for all investments within energy. That includes a forecast for energy prices and costs of CO2 as well as all harmful emissions. The discount rate is 5 %, but it is considered from the political level to reduce it to 3-4%
When we compare two energy solutions in the energy planning, we therefore estimate the net present value (NPV) of all costs including the environmental costs at the level of the society in the life time. The solution with the lowest total costs will therefore be the preferred solution, and moreover, it will have the highest score on the two first criteria of sustainability.
But what about the social criteria? Can we measure it? For urban solutions, yes, because our energy choice in urban areas has an impact on the rest of the local community. If we chose an energy solution, which has the lowest cost at the level of the local community, our score will be higher on the social criteria.
Therefore, the energy solution with the lowest NPV costs at the national level (including environmental costs), and at the level of the local community will be the most sustainable.
If our solution also has a low environmental impact for other factors (noice, visual impact etc) our score will be even higher.
Case of Carlsberg in Copenhagen
We have analyzed the sustainability of the energy solutions for the new city district Carlsberg in Copenhagen and came to a rather clear conclusion.
The most sustainable energy solution for all 3 criteria will be
to connect the buildings to the urban district heating infrastructure based on CHP and renewable energy instead of investing in building level heat pumps and solar heating
to connect to the power grid and invest in off shore wind farms for electricity production instead of investing in local building level solar panels or wind energy turbines.
By joining the rest of the city and sharing the district energy infrastructure with the other energy consumers, we increase the total efficiency and we lower the heating costs a bit for all heat consumers in the city. That is both a symbolic and a concrete example of social responsibility.
Besides our analysis showed that the normal Danish standard for insulation would be more than sufficient. As the normal level of insulation is sufficient, the additional benefit from more indulation would have very little effect. Therefore from a sustainability point of view money could be spend more intelligent on more proditable measures.
Outside urban areas
In farm houses we find another solution. Individual heat pumps and individual solar heating will be the best energy solution, and it might be an idea with a wind turbine, which do not disturb neighbors.
In the mountains
In the Norwegian mountains we come to a third solution. For buildings far from public road there is no connection to neither district heating, water, waste water or electricity. The most sustainable solution seems therefore to be solar cells and batteries, which combined with a wood stove can provide us with all necessary energy services.