Energy and buildings

Energy consumption

The world final energy consumption amounts to 279 EJ in the year 1994. Hereof, 117 EJ, which is 42%, fall to households and small trade, mainly for heating and cooling buildings. Figure 1 shows the distribution of the final energy consumption on the different sectors. A large share of the world wide energy consumption is caused by households. This embodies a vast potential for energy savings, because an optimisation of energy use for households has not yet taken place. The reason for this is the long service life or building lifetime, respectively. This period is much longer than for other goods, especially in technology. A building is technologically outdated at the earliest after 30 years. This point of time is reached for a car after 10 years at the most, and even after 2 to 4 years for computers. This leads to a comparatively long time for the distribution of newly established energy efficient standards by new buildings. Through additional measures for an energy retrofit in conjunction with renovation of the building stock, a huge potential results for the aspired saving of resources.

Figure 1: World wide final energy consumption by sector: Households and small trade (42%), industry (31%), transport (22%) and non-energy use (5%). The corresponding primary energy consumption amounts to 389 EJ [5] in the year 1994.

Heat balance of buildings

The heat balance of a building includes all sources and sinks of energy inside a building, as well as all energy flows through its envelope. This envelope encloses the volume which is kept above a set temperature (in general 20 ºC) for all weather conditions by the use of heating energy. The extend of all heat flows, which do hereby occur, is either dependent on external or internal influence factors (weather, user). These heat flows can be arranged into five categories:

(1) Transmission losses LT are those amounts of heat, which flow through the building envelope from inside to outside by conduction or heat transfer, respectively.

(2) Ventilation losses LV are caused by exchange of warm indoor air with colder outdoor air. The user independent air exchange is through joints by infiltration or exfiltration, respectively. In addition, room air can be exchanged through open windows or by a mechanical ventilation system. Ventilation is indispensable, up to a certain extend, to assure the hygienically necessary air exchange rate.

(3) Solar gains GS are irradiations of solar energy through windows and other transparent or translucent constructional elements. Also added to the solar gains, is that part of the solar heating of the opaque building envelope, from which the indoor area benefits.

(4) Internal gains GI are heat outputs from persons, appliances, computers and other electric devices, as well as from illumination.

(5) Heating demand H is exactly that amount of energy, which is necessary to maintain the desired room temperature by compensating the excess of losses (1 and 2) compared to the gains (3 and 4).

Figure 2: The 5 elements of the heat balance of a building.

Figure 2 shows a diagram of these elements of the heat balance. The gains and losses are specified for a certain period of time (e.g. one year). Division of this value by the corresponding area of heated space in m2, gives all heat flows (1 to 5) in the usual unit for the (floor space) specific energy demand for heating: kWh/(m²·a). The allocation of the transmission and ventilation to gains or losses depends, strictly speaking, on whether the outdoor temperature is higher or lower than the room temperature. If the gains exceed the losses for a longer period of time, the desired indoor temperature would be overstepped. Instead of heating, cooling would be necessary for the balance. This case occurs in summer and is treated separately. In northern and middle European climates, in general, the winter case is considered. However, it can happen in winter, but especially in the intermediate times (autumn, spring), that the set temperature is sporadically exceeded by high solar or internal gains. The total monthly internal and solar gains are, however, not to 100% effective for heating. Therefore, they are rated with a utilisation factor FU < 1. Typical values for the yearly mean value are in the range FU 0.5 to FU 0.9, depending on the heating energy demand and the kind of construction. For an extended period of time (several month, heating period) the changes of the stored energy in the building mass, indicated by the mean building temperature, are negligible, and the energy balance is [1]:

This means, the heating energy corresponds to the sum of the losses, reduced by the utilised part of the gains. Figure 3 shows 2 examples for a heat balance. The example for the building stock (20s to 80s of the last century) has an energy demand for heating of 284 kWh/(m²·a). The example of a low energy house (see below) with high insulation standard and ventilation system with heat recovery has a heating energy demand of only 66 kWh/(m²·a).

Figure 3: Two examples of a heat balance (for Germany). On the left, a typical balance for the average building stock. On the right, the balance of a low energy building.

Building standards

The simplest energy standards for buildings are referring to the specific heating energy demand (see Energy). Typical values for the specific heating energy demand for the building stock are in the range 100 to 300 kWh/(m²·a). By appropriate technical effort the heating energy demand can be strongly reduced. The following standards or technologies, respectively, have been introduced in the German-speaking area:

Low energy houses (Niedrigenergiehäuser) have values for the heating energy demand of about 50 to 70 kWh/(m²·a), (see Figure 3). In Germany, this appellation is used for buildings, which have an about 30% lower heating energy demand than allowed by the building code for new buildings (WSVO'95), which was in force between 1995 and February 2002.

So called "3 litre houses" have a specific heating energy demand of (somewhat less than) 30 kWh/(m²·a). This corresponds to an oil demand for heating of about 3 litre/(m²·a), (see Energy, see Table 2). This classification does only make sense, if the heating energy demand is actually covered by fossil fuels. If a heating based on electric energy is used, the specific primary energy demand is increased by a factor of 3, because of the losses caused by the electricity generation. In such a case, a heating energy demand of 30 kWh/(m²·a) would correspond to a primary energy demand of 9 litre/(m²·a).

Passive houses (Passivhäuser) have an extreme high insulation level, use ventilation systems with very efficient heat recovery, and utilise solar energy by energy efficient glazing or windows, respectively. They exhibit a typical heating energy demand in the region 15 to 25 kWh/(m²·a).

Further technological measures, like seasonal solar thermal energy storages to supplement the heating, or photovoltaic panels to partly cover the electricity demand, lead to self-sufficient houses, which have in particular a heating energy demand reduced to zero [2]. The cumulated energy demand (see Energy) and the over the whole lifetime cumulated costs of such buildings, which have rather the character of research and demonstration projects, is currently still higher than for the multiple realised passive houses.

Factors of influence

The heating energy demand of a building is influenced by the climate conditions at the location, the immediate building surroundings, and the behaviour of the users. The most important climatic influences are outdoor temperature and solar radiation. The heating energy demand for a heating period depends on the level of the outdoor temperature as well as the length of this period. Both influence factors are described by the Heating Degree Days (HDD). This is based on a indoor temperature of 20 ºC and on a heating temperature of 15 ºC, i.e. heating is necessary only on days, where the outdoor temperature is less than 15 ºC. For every day, on which this condition is fulfilled, the difference between 20 ºC and the mean outdoor temperature is summed. This gives the heating degree days on the basis (20/15), denoted HDD15. The unit of the heating degree days is Kelvin-days (Kd). For buildings with high insulation level (low energy buildings) a heating temperature of 12 ºC is assumed. This gives the heating degree days HDD12 on the basis (20/12). The transmission and ventilation losses are directly proportional to the heating degree days, which can vary significantly for different locations (and also for different years). In Table 1 values for the mean heating degree days of a year are compiled for different locations in Europe. The losses for a certain building depend on the location and the corresponding heating degree days. For example, according to Table 1, the transmission and ventilation losses for a building in Stockholm are about 62% higher than for the same building in Dijon. Also, a variation of the desired indoor temperature leads to a change of the losses, which is described by the related heating degree days. For, e.g., 200 heating days per year and an increase or decrease of the indoor temperature by 1 ºC, the heating degree days change 200 Kd in both cases.

A further influence factor for the heating energy demand is the solar radiation (see Renewable energies). The higher the irradiation at the location of the building, the higher the solar gains can be. The influence is, however, much smaller than that of the indoor and outdoor temperatures. At buildings with high glazing to wall ratio (more than 40%) or large areas of transparent insulation (see Thermal insulation), the influence of solar radiation can become significant. For the dependence of the solar gains on the global radiation, the individual shading situation, e.g. by neighbouring buildings, the typography and vegetation plays an important role. Finally, the influence of the wind on the heat losses is noteworthy. The pressure distribution on the facade, generated from the wind velocity and its direction, depends on the character of the immediate surroundings (typography) (see Building and Environment). It effects the total ventilation losses of a building by infiltration and exfiltration through joints in the building envelope. For modern buildings, which are largely wind- and airtight, this influence is, however, inferior.

In addition to the above mentioned set temperature, the building user influences the heating energy demand of a building by his ventilation behaviour. Inadequately strong window ventilation can push the heating energy demand to the limit of the furnace performance. Even further ventilation losses lead to a sinking temperature level, resulting in discomfort. From the point of view of energy saving, several short (5 to 10 minutes) and strong ventilation occurrences with heating switched off, are more prosperous than longer lasting ventilation with slightly opened windows. For the accurate adjustment of the hygienically necessary air exchange rate, and, therefore, to optimise and limit the ventilation losses, a mechanical ventilation system is ingenious (see Ventilation).

Building services

The role of the building services is to cover the demand of the user for water, electricity, heat and increasingly also fresh air. The building services, relevant for energy, are mainly heating system, domestic hot water and ventilation system. The different building standards have discriminative requirements and possibilities with respect to the building services. In houses of the building stock, a powerful heating system in necessary to cover the heating demand and to make the heating comfortable. Here, usually a gas, oil or electric heating system is used, even in smaller buildings. In buildings with higher insulation standard (low energy houses), energy sources with lower energy density, like wood heating (e.g. pellets), can also be used. Moreover, the temperature of the heat transfer can be lower (< 35 ºC), so that a supplemental solar collectors are energetically favourable. In addition to the high insulation level in passive houses, a ventilation system with highly efficient heat recovery is utilized to save expenses for a conventional heating system and the corresponding infrastructure (oil tank, gas connection, furnace room, chimney, separate heat transfer system, heat exchanger). The low heating energy demand is covered by a small heating module, e.g. by liquid gas or electricity, or by an adequately dimensioned heat pump [4]. As heat distributor, the ventilation system is used (which is installed anyway). The domestic hot water is usually provided in passive houses by a solar collector, which is supplemented by the existing heating system. The building services of self sufficient houses eventually have to supply also the (small) rest of the heating energy demand by additional technical effort. This can be achieved by seasonal repositories, which store the solar energy over collectors during the summer into a central tank, laid out often for several buildings (storage medium generally water). This energy is used as heating energy in winter. Hot water storage tanks can store 60 to 80 kWh thermal energy in each m³ of water [5].

Grey energy

Energy is necessary for the construction and disassembly of a building. This energy is called grey energy, and contains energies for the manufacturing and processing of the building materials, their transport and disposal. Together with the energy for covering the heating energy demand during the lifetime of the building, this results in the cumulative energy (see Energy). Only this measure is suitable, to make an evaluation of the sustainability. The grey energy of a residential building, distributed over the lifetime of its different components, is roughly 30 kWh/(m²·a), [6]. This is a significant part of the energy, which is used for heating during the usage of the building. The higher the constructional and technical effort of a building, the higher is this fraction.

Literature

[1] European Comitee for Standardization: EN 832 - Thermal performance of buildings, German version: DIN EN 832 - Wärmetechnisches Verhalten von Gebäuden, Beuth Verlag GmbH, Berlin, 1992.

[2] K. Voss (Hrsg.): Konzeption und Bau eines energieautarken Solarhauses, Schlußbericht zum BMBF Projekt; Fraunhofer IRB Verlag, 1997, ISBN 3-8167-4610-1.

[3] METEOTEST: 'METEONORM, Edition '97, Global Meteorological Database for Solar Energy and Applied Meteorology', Version 3.0, (Bern 1997), http://www.meteotest.ch.

[4] BINE Informationsdienst: Neue Wärmepumpen-Konzepte für energieeffiziente Gebäude, BINE projektinfo 14/01, Fachinformationszentrum Karlsruhe, 2001, http://www.bine.info.

[5] BINE Informationsdienst: Langzeit-Wärmespeicher und solare Nahwärme, BINE profiinfo 1/01, Fachinformationszentrum Karlsruhe, 2001, http://www.bine.info.

[6] Wagner et al.: Ökologische Bewertung im Gebäudebereich, Endbericht, AG Solar NRW (North-Rhine Westphalia), Project No. 262 205 99, Essen, Germany, 2002.