Cost Optimal and Nearly Zero-Energy Buildings (nZEB)

Nearly zero-energy building (nZEB) requirements can be seen as a major driver in construction sector for next years, as all new buildings in EU are expected to be nearly zero buildings from 2021. In many countries, present energy performance minimum requirements have not been able to follow increasing energy prices—that has been revealed by cost optimality analyses showing the fact that requirements lag behind and are not able to provide minimal life cycle cost of construction and operation of buildings even with reasonably short life cycle periods used. These two new terms, nZEB and cost optimal energy performance, were launched by energy performance of buildings directive recast (EPBD recast) in EPBD (2010). EPBD requires that energy performance minimum requirements will be shifted to cost optimal level as a first step towards nZEB buildings. Member States have to define what nZEB for them exactly constitutes. It is easy to realize the problem that various definitions of nZEB may cause in Europe if uniformed methodology will not be used. In this book, the latest information on technical definitions, system boundaries and other methodology for energy performance calculations, as well as description of technical solutions, based on nZEB building case studies can be found. This could help all persons needing to be aware or working with energy performance of buildings.

How to define nearly zero-energy buildings? This is one major question for member states when implementing EPBD directive. The directive requires that energy performance of buildings, as well as the definition of nearly zero-energy buildings, should be expressed with a numerical indicator of primary energy in kWh/m² per year. Therefore, if a national energy frame (energy calculation methodology and minimum requirements) is not based on primary energy, first a new energy frame/methodology has to be developed and implemented in a building code in order to be able to implement the directive. This could be a major effort in countries where minimum energy performance requirements have been based on the requirements of building components or on energy need or on delivered energy. In this chapter, energy flows needed for the primary energy indicator calculation are described based on REHVA and CEN definitions, Kurnitski REHVA Report No 4, (2013), prEN 15603 (2013). A specific issue of nZEB buildings is accounting the positive effect of on-site and nearby renewable energy production, which needs to be included in the energy frame. Energy frame is described with system boundaries for each energy calculation step, starting from the energy need to the final system boundary of the delivered and exported energy, which allows us to calculate primary energy indicator and renewable energy contribution with national primary energy factors. Because of complicated definitions and system boundaries, calculation examples for all main cases are provided.

At the moment, there are already some official definitions of nZEBs available at least in Denmark, Estonia, and France, but most of the Member States intensively work with national definitions and plan for nZEBs. In the following, the situation in some selected countries is reported. At the end of the chapter, the issue of comparison of national requirements is discussed and comparison results of some countries based on the data of January 2013 are shown. Some harmonization in minimum energy performance requirements can be seen.

EPBD recast requires Member States (MS) to ensure that minimum energy performance requirements of buildings are set with a view to achieving cost optimal levels using a comparative methodology framework established by the Commission [1]. Cost optimal performance level means the energy performance in terms of primary energy leading to minimum life cycle cost. MS had to provide first cost optimal calculations to evaluate the cost optimality of current minimum requirements due March 2013. After that, MS need to revise calculations and to submit reports to the Commission at regular intervals, which shall not be longer than 5 years. Cost optimal methodology is intended for the minimum energy performance requirements, but as well defined methodology, this can be used also in any construction project in order to find most cost optimal solutions. In the following, a systematic and robust procedure to determine cost optimal energy performance solutions is discussed.

This chapter summarises the factors which should be considered in the design and operation, focusing mainly on room temperature, indoor air quality, ventilation, moisture and humidity, noise and lighting. The information is mainly based on the European Standard EN 15251:2007, but as this standard does not cover all indoor environmental factors to be considered in low-energy building design also other sources are used. Many certification systems for buildings include in the evaluation criteria both energy use and the quality of the indoor environment. Thus, it is desirable to develop systems and solutions which lead to high-quality indoor environment with low energy use. The designer shall always document design criteria for the indoor environment; these criteria shall be available with the energy use data when renting or selling the building space. It is also recommended that design values for the indoor environment and indicators for the environmental comfort are included in the energy certificate and displayed with actual values for the energy use. This chapter describes also the difference between target values for dimensioning of systems and energy calculations. Different approaches for mechanically cooled buildings and buildings without mechanical cooling are introduced, and precautions are given for the latter if to be applied in low-energy buildings.

Energy use of buildings is strongly affected by the climate the building is located. Some measures are effective in all climates, but attention to energy balance components and proper solutions depends on climate. An office building case study is used to show the performance in all climates, temperate, Mediterranean, cold and tropical described with Paris, Rome, Stockholm and Bombay weather data. It is shown that energy performance can be strongly improved with energy-efficient building envelope elements especially for windows and solar shading, modern lighting system with intelligent controls and optimal HVAC system with very efficient heat recovery, good chiller design and a high-temperature room-conditioning application. When building is located in Mediterranean or tropical climate conditions, significant part of energy use comes from cooling/drying of supply air, stressing the importance of corresponding solutions. Energy efficiency measures are evidently important design issues, to be tackled already in very early stages with integrated design. This applies also for architectural competitions. The problem is that if energy performance targets will be applied after architectural competition, this might be too late, and in worst case, the whole proposal has to be redesigned to meet the targets. To avoid such problems, energy performance targets are to be included in the competition brief among all other targets. It is discussed how energy performance targets can be included so that they will lead to integrated design from very first steps, but unnecessarily, complicated and detailed analyses can be avoided. Two possible approaches, one based on simple indirect indicators requiring a minimum calculation effort and another based on energy simulations, are discussed. A case study example with the application of the second approach is reported.

nZEB buildings generally require integrated design in order to achieve design targets economically. Decisions and choices in early design stages may be expensive or even impossible to fix later if have not been successful. Massing not supporting energy-efficient design or lack of space for technical systems is typical example of potential drawbacks. It is important continuously to follow that design targets can be met. In early stages, rules of thumb and some key parameters can be used for indirect assessment, which is the method until first energy simulations can be run. Next step is to be sure that planned technical systems can be fitted in the building—there has been enough mechanical space and proper locations enabling energy-efficient design. These and other important milestones in the early stage including fenestration design, shadings and daylight are discussed in this chapter. It is not enough to design a good nZEB building, but it has to be done in a way that the building can be also operated as nZEB building. In majority of projects, designed room layouts will change already during construction, because of clients’ needs. Therefore, the HVAC systems must adapt to changed loads and partition wall locations. To enable flexible space use and adaptive systems, special considerations and the use of room modules are needed, that is, the last but not least issue discussed in this chapter.

There already exist pilot projects across Europe, which may be called nZEB buildings. They are not easy to compare because of variation in the definitions and performance levels—nZEB definitions have not yet been available when these buildings have been designed. It is important to check which energy uses are included in the calculated and measured energy performance and are the results reported as delivered or primary energy. Tenant’s electricity (appliances, lighting) is often not included. Simulated primary energy is typically between 50 and 100 kWh/(m² a) for high-performance nZEB buildings with on-site renewable energy production if all energy uses are included. Such buildings may be extremely complicated, e.g. control of mixed mode ventilation or integrated energy supply solutions with storage and many operation modes. “Overkill” complexity may have implications on operation and maintenance as nobody cannot manage systems, which are not easy to control and operate. Another trend that can be seen is simple and reliable solutions based on high-performance components and careful system design and fitting with building properties. In the following, five nZEB buildings across the Europe are reported. Three of them are with measured energy data, indicating that strict targets are not always easy to achieve; however, some deviations have been caused also by unrealistic energy calculation input data. Technical solutions used show that there are many alternative ways to achieve high performance; however, strong differences in design bases and basic solutions in similar climate may also show that optimal solutions are always not known or found. Four first case studies are mostly focused on description of technical solutions, but the last one also describes the tuning of systems needed in the first year of operation to achieve the targets.