Method for use of economical optimization in design of nearly zero energy buildings

Nearly zero energy buildings are to become a requirement as part of the European energy policy. There are many ways of designing buildings to become nearly zero energy buildings, but there is a lack of knowledge on how to do it in the most economical way. Therefore it is very important to develop methods of designing such buildings in an economical optimized way.

A method of finding the economical optimal solutions has been set up based on use of the cost of conserved energy for each main building envelope part and building service system. By use of information on construction cost and a simple model of the energy use for each component, a function is set up that represents the relation of the marginal cost of conserved energy and the energy use for different qualities of the component.

For building envelopes, the energy use will be the heat loss during heating season, and the main quality parameter will be the insulation thickness, but for windows the energy use will be based on the net energy that takes both solar gain and heat loss into account. The optimal mix of solutions for the whole building is found by selecting building parts with the same cost of conserved energy.

The constraint is that the total energy performance of the building is fulfilling the requirements. To find the solution automatically the Solver function of Excel is used and compared to existing programs the method overestimates the energy use with 2.3-8.3 %. A case example shows how the method with success can find the solution for a typical Danish single family house with the energy performance requirement for 2015.