Professor Marc Frère of the University of Mons Research Institute for Energy looks at the arguments for creating low energy demand buildings
In Europe, the building sector is a major contributor to energy consumption and, hence, is responsible for 36% of the continent’s greenhouse gas emissions1. In a country like Belgium, space heating needs are mainly covered by fossil fuels (90 TWh/year: 21.7% of the total fossil fuel consumption)2.
Construction techniques and energy conversion technologies that are currently available could considerably reduce the environmental impact of the built environment. The concept of very low energy or passive building makes possible the reduction of the annual heat demand (from 130 kWh/(year m2)) – average space heating demand for existing buildings in Belgium to 15 kWh/(year m2)).
Furthermore, low heating/cooling power may be delivered at quite low/high temperatures, which increases the energy conversion efficiency and/or the renewable contribution of heating and cooling technologies.
There is thus a kind of positive chain reaction when promoting the development of low energy demand buildings. As an example, when virtually transforming all the Belgian building stock into passive buildings heated by high performance air-to-water heat pumps (seasonal performance factor=3.1), the fossil fuel consumption for space heating could be reduced to 7.8 TWh/year (considering a heat distribution efficiency equal to 0.97 and a non-renewable primary energy to electricity conversion efficiency equal to 0.4) instead of 90 TWh/year. The huge gain between the two situations is a combination of reduction of the heat demand and an increase in the global conversion efficiency from non-renewable primary energy to heat.
The European Union promotes the transition towards low energy buildings allowing the use of technologies with a renewable contribution (Energy Performance of Buildings Directive – EPBD – and its recast3).
The idea behind Net Zero Energy Buildings (NZEBs) is that the total annual energy need of the building must be counterbalanced by a renewable production (local production). The electricity needs for ventilation, lighting and domestic appliances may be covered by PV panels. In the case of using a heat pump for space heating and DHW production, the additional electricity consumption may be covered by an extra PV surface area, which is possible for very low heat demands (e.g. passive houses). The generalisation of such a concept could considerably reduce the fossil fuel consumption.
This concept is pushed up by European regulations. However, some major issues still need addressing before these policies can have a real impact on the fossil fuel consumption of buildings:
- EPBD regulation mainly applies to new constructions and heavy refurbishment projects. In a country like Belgium, the building stock is growing with a rate of 0.6% per year due to demography and growing share of services in the economy. The demolition rate is only 0.2% per year, leading to a weak/low impact of new regulations on the building sector energy consumption.
- NZEB concept increases the investment costs.
- NZEB concept, as well as specific policies for promoting the production of green electricity, will lead to the electrification of space heating (through heat pumping). When considering annual energy balance, the above-explained example shows a striking positive impact in terms of fossil fuel consumption. However, it requires a dramatic increase of the electricity production capacity (several GW for Belgium). The mismatch between green electricity production and consumption (namely for space heating) may also require either storage capacities or additional conventional power plants.
To correctly address the last issue and its impact on power generation, it is urgent to promote the concept of nearly energy autonomous buildings or city districts.
In such a concept, a building or a city district would be equipped with green energy production capacities, conventional energy production capacities, as well as energy storage capacities, in such a way that the energy needs are mainly covered by renewable energy sources, with a reduced impact on the global energy network.
It means that, in the case of low green energy production, the needs may be mainly covered by the energy that was stored during periods for which green production was exceeding the needs. This concept may be supported by the following arguments:
- The built environment is probably the unique sector for which technical solutions exist for drastically reducing the energy demand and introducing renewable energy resources. This fact probably justifies a specific energy policy for the building sector with no or limited impact on the other sectors.
- The energy demand of buildings has some specificities. A part of the energy needs (heating and cooling) has a seasonal occurrence. Energy needs cover electricity needs, heating and cooling needs; energy conversion technologies associated with energy storage capacities allow flexibility. For example, exceeding green electricity may be either stored in batteries or converted into heat using a heat pump – a heat storage may be used if there is no heat demand; CHP provides at the same time heat and electricity; cooling needs may be covered by sorption machines which are thermally driven; thermal energy may be produced by solar collectors; heat or cold storage will add flexibility to the system.
- These examples show the complexity of the energy demand profile of buildings may be a source of big perturbations on the energy grid if buildings are considered as pure energy consumers – but could be at the origin of the development of efficient energy systems when such systems are specifically designed for maximising the local use of renewable resources.
- The concept of a nearly autonomous building gives the responsibility to the building sector and building users in terms of contribution to the energy transition. It allows an integrated approach of energy production/storage/distribution systems and buildings taking into account local specificities in terms of climate conditions and local renewable resources.
- When considered at the district level, this concept makes possible large-scale investments (district heating, geothermal wells).
- Promoting the local use of locally produced green energy may contribute to create social links and to re-centre energy policies at the local level.
Such a concept has already been experienced4, 5. It appeared from existing case studies that:
- Extra investment costs must be covered by public bodies, highlighting the need for a new economic model.
- There is a wide variety of technical solutions for combining technologies in order to reach high levels of onsite consumption of locally produced energy.
- Electricity and heat storage, as well as their collaborative working, is a key R&D topic to focus on.
- Education programmes that eliminate the barriers between disciplines related to energy should be encouraged (both at the scientific and technical levels).
This paper was written in the framework of the RESIZED project (ERA CHAIR project funded by EU) devoted to the efficient use of energy in city districts.
Please note: this is a commercial profile
University of Mons,
Research Institute for Energy
Tel: +32 65 37 42 06