PAZO DE SOBRAN.

In accordance with selection criteria of energy saving measures in residential buildings, technical optimisation plays the primary and direct role in reducing energy consumption and costs, and improving the indoor thermal comfort. Technical optimisation measures involve two main assignments: the constructional retrofitting and the efficiency improve- ment of building energy system. These measures could be implemented based on the sta- tus quo of building construction, on the occupants’ requirements and the affordable in- vestment (including initial costs, recurring costs, and risk and mitigation factors), in form of individual measure or measures package to meet actual demand. Based on the project experience of building energy efficiency some measure packages could be summarized as follows:

- Operative management. Energy system management during the building opera- tion phase refers in this research to optimising the heating curve for rational supply temperature of heating system, regulating DHW supply and installing water sub- meters, as well as monitoring and reporting. Based on an effective operative man- agement, it aims to achieve an optimized demand response scheme that could shed loads in response to the actual energy demand and the real energy price condition through using a series of dedicated control systems (EU-Project: aFTeR).

Currently, building management system (BMS) can also follow and control the running time and settings of heating system and help monitor and regulate some of the parameters (the supply temperature, and return/flow temperature of heating system, air flow rates of the ventilation system, etc.). In addition, operative man- agement is strongly influenced by occupant behaviour, therefore energy monitor- ing instrument for occupants (e.g., web-tools, smart meters) is an interesting so-

116 _{Household energy poverty is defined as the level of a household income below the minimum energy}

costs that are necessary to achieve a satisfactory living condition with a dwelling (Bergasse et al. 2013, Aranda et al. 2017), especially for households living in social housing buildings.

lution to inform and empower occupants and thus contribute to technological op- timisation measures.

- External insulation of building construction. Heating energy cost is considered as the most part of household energy expenditure, in particular in North- and Cen- tral European countries, also in North China. In contrast with electricity consump- tion largely affected by appliance performance and occupant behaviour, the heat- ing energy efficiency is primarily influenced by the insulation situation of external construction elements, e.g., façades, windows and roofs etc.

Façades are subject to heavy stress due to the increasing humidity and temperature fluctuations, which lead to not only thermal energy loses also mould and fungi within the indoor area. A study by University Jena, Germany found that the mould infestation of about 60% of 15,000 investigated apartments was due to building materials, which combine with structural components to determine the façade in- sulation performance. That is to say, the thermal characteristics of building mate- rials (e.g., thermal conductivity λ [W/(m•K)], specific heat capacity c [J/(kg•K)], and thickness of wall or thermal insulation layers etc.). While in 30% of cases, the combination of incorrect ventilation behaviour and building materials was the only cause of mould formation (Weber and Sprungala 2012, p.100).

In view of rising temperatures, it is advisable to pay attention to a good summer heat protection in the façade thermal insulation layers. This concerns in particular the roof insulation, where the insulation materials must have a good heat storage capacity, thus to delay the heat flow inside the rooms significantly. The insulation of building façade is considered one of the most effective insulation strategies for both new and existing buildings. In particular, for existing buildings, there are two choices to optimizing façade insulation: external or interior insulation solutions. By contrast, external insulation does not take up additional living space and keep the indoor areas being spared from the renovation work, however, interior insula- tion could save the cost and time for scaffolding but at the expense of reduction of indoor volume.

Leaky windows (e.g., leaky window frames and one-panel glazing) could cause about 20-25 per cent of the heating energy escapes from heated indoor areas to outside, almost same like uninsulated exterior walls. In order to reduce this ther- mal loss path, simple measures are sufficient in the first step. For example, replac- ing the sealing strips of windows, the occupants have the choice between sealing tapes made of foam and rubber profiles in Germany. Foam seals are relatively cheap (about 7-10 Euros/meter), but only hold one heating season. Rubber seals or seals made of similar materials are much more expensive (i.e. cost between 15- 25 Euros/meter) with a lifespan of four to eight years. Meanwhile heat escapes through the window glasses if the glasses have a weak insulation character. In

Germany, there are still many windows with single glazing. A quick and relatively simple alternative to optimizing the thermal character is to glue windows with an insulating film, which costs about 10-20 Euros depending on the size of slides. A complete replacement of windows is frequently unavoidable to reduce the energy loss, usually in the case of old buildings that have an extremely poor insulation value. A new standard window for buildings in Germany costs in general about 300 Euro per square meter of window only for material expenses, or 500-600 Eu- ros including installation fees, according to the report from NRW Energy Agency117. Window with top quality would be more expensive. The lifespan of windows with state-of-the-art insulation lasts usually 15-20 years.

In addition, it is also useful to optimize the insulation of the roller shutter casings with a high-quality insulation material (e.g., polyurethane or phenolic resin), which is a simple and effective solution. It is important to note that the joints should be after installation of new insulation items well sealed with permanently elastic acrylic sealant.

Roof/attic insulation is suggested implemented from the outside if the attic is in- habited. However, if there is enough space in the attic, an inside roof insulation can be easily realized with an insulation between or under rafters in steep or flat roofs. The cost of roof insulation varies greatly depending on the roof shape, the condition of roof, the selected type of roof insulation and insulation materials. Basement insulation depends usually on whether it is heated. If the basement is designed as a cold space, the insulation of basement ceiling is recommended, as well the hot water pipes and heating pipes should be well insulated too. If the basement is heated and serves as a useful part of an apartment (e.g., work room or private library etc.), then the basement floor and walls should be well insulated. Similar with other construction components the concrete insulation measures, i.e. from outside or inside, depend on the actual condition, requirements and budget. The external insulation improves the winter and summer indoor living comfort thanks to the reduction of cold wall effect in winter and the reduction of solar gains in summer.

- Running maintenance. Comparing with other optimisation solutions, technical maintenance should be a low-investment measure. It includes regular maintenance of heating system with a proper level for achieving an efficient heat distribution inside buildings, complementary insulation of heating system components (e.g., pipe) for reducing thermal loss by transporting process, regular clearance of boiler and hot water tank (e.g., calcium removing) for recovering the efficiency of DHW

supply, installing water filter and water pre-processing system to maintain the fa- cility in a sustainable (e.g., economic and ecological) state.

Although maintenance is relatively cost-effective and time-effective optimisation option, it is at most to reduce the energy waste instead of saving energy or recov- ering efficiency and thus is recommended as energy saving measure for a short- term instead of long-term strategy.

- Replacement of systems. Except for poor tightness building façade, heat losses appear in most existing residential buildings due to old heating systems (e.g., boiler, pump). In traditional boilers, the combustion of energy fuel produces hot gases with potential heating energy that is transferred to the water circulation in the system thanks to the action of the exchanger. During this process, heat losses provoked due to the temperature difference between the hot gases and the air of the combustion and stream contained in the gases, which are blown into the at- mosphere and wasted. In contrast, the condensing boiler could take advantage of the potential heat contained in hot gases. This process consists of using the latent heat of the vapour and distributing it into the water of heating system. As a result, the condensing boiler will use and extract waste energy resulting from the com- bustion process to space heating and preheating the return water before it is heated (EU-Project: aFTeR).

Comparing with other ESMs, system replacement is a costly and complex meas- ure to improve building energy efficiency. Several factors have to be considered comprehensively, such as the existing heat energy production system, the lifespan of system, and the affordable budget etc.

- Integration of renewable energy. The most popular integration of renewable en- ergy into residential buildings includes, for example, solar water heater for DHW supply, PV panels for electricity generation, and geothermal energy as the most reliable renewable energy source (Rezaie et al. 2013) for space heating and cool- ing, if the average COP value of geothermal system is rough 4. Depending on requirements and budget, these integration approaches could be implemented in- dividually also combined as a hybrid system.