The purpose of retrofit is to reduce both energy consumption and carbon emissions of buildings, however, minor consideration is given to the impacts it has on the internal environments for the occupant. Considering the various conditions around the continent, buildings constructed in one location are likely to vary from another, and the measures used for retrofit will reflect this. Additionally it would also report that most assumptions are based on computer models, whose input variables are easily scrutinised by anyone educated in the relevant knowledge.
In the Holistic Energy-efficient Retrofit of Buildings (HERB) project, new retrofit technologies are being developed that can be applied to buildings with the aim to save 80% on energy consumption and 60% on carbon emissions, this being combined with the retrofit of the houses mentioned and a range of pre and post monitoring. Opportunity is therefore present in the project to undertake comparison between models generated that predict energy consumption and internal comfort conditions, pre and post retrofit, and compare these with actually measured data in the houses to be upgraded. The overall aim of this research is the production of a Holistic approach to retrofit, this incorporating all factors mentioned. Currently retrofit is undertaken piecemeal, meaning measures are installed independently with minor consideration on how their performance is affected by other factors in a building. Through considering all factors in the holistic approach, a recommendation will be provided that resolves these issues, as all technologies installed will be designed to operate harmoniously with high efficiency. In HERB work package 1 the stages of the holistic approach were postulated, the workflow direction of this is shown.
The holistic approach, as postulated in HERB work package 1
This methodology centres on a central simulation environment, where buildings are constructed in software packages that model energy consumption (WP1 – T1.2) and thermal comfort (WP1 – T1.3). Modular inputs are inserted into this system based on the new HERB innovations (WP1 – T1.1), with these ultimately affecting the final output. There is no limitation to the number of inputs that can be used in the process and those used in each location were stipulated in the project document. Output from the central system is then analysed economically in relation to social and financial impacts (WP1 – T1.18). The purpose of the system is the optimisation of an eventual retrofit recommendation. By considering a project holistically in relation to technology, practicality, location, comfort, energy consumption, and cost, the recommendation will be most suitable for the case it is applied to. Additionally, the process is not limited to any specific software packages; therefore the process can be used by any party (as long as limitations and assumptions are clearly specified in the outset).
Fundamentally, this is centralised around the targets of the project that include:
- A cumulative annual energy savings of at least 80% measured against building performance before retrofit;
- At least a 60% reduction of CO2 emissions;
- A global energy consumption (excluding appliances) of 50 kWh/m²/year while reducing peak loads against the values measured before retrofit;
- At least 80% energy saving for lighting over the average consumption of the installed base;
- A user acceptability and long term continued efficient operation; and
- A pay-back period of between two and five years compared to current state of the art, depending on the type of technology and solution.
Information on the property was taken from the surveys, and according to the requirements, interventions were proposed to be implemented. Through simulations, these were analysed in their current condition and in the different intervention scenarios with respect to energy consumption and comfort. On completion, results were analysed with respect to cost / benefit to determine if the retrofit process was economically viable. If it was not acceptable, the process was repeated to find feasible alternatives. On identification of an acceptable solution, the final recommendation was made.
Technologies in Force
Vacuum Tube Window
In order to reduce the heat loss through glazed surfaces the concept of a vacuum tube window has been proposed by University of Nottingham. In this concept several glass tubes with vacuum inside are sandwiched between two glass surfaces. The non-vacuum parts of the structure are filled with a gas of a low thermal conductivity, i.e., argon. The window has to be sufficiently lightweight. Therefore the thickness of the window was limited to about 80 mm. Two different geometries were studied:
- Double tube window where the vacuum is between two co-centric glass tubes;
- Single tube window where the vacuum is formed inside a single glass tube.
These two geometries were compared. In addition further optimization was made for the geometry having better thermal properties. The thermal properties were quantified with the U-value of teh window which represents the overall thermal resistance of the structure.
“Double tube geometry”
“Single tube geometry”
Self Cleaning Coating
The technology of self-cleaning coatings has developed rapidly in recent years. As a commercial product, their potential is huge. Because of the wide range of possible applications, from window glass and cement to textiles, self-cleaning coatings may become an important labour-saving device.
The field of self-cleaning coatings is divided into two categories: hydrophobic and hydrophilic. These two types of coating both clean themselves through the action of water, the former by rolling droplets and the latter by sheeting water that carries away dirt. Hydrophilic coatings, however, have an additional property: they can chemically break down adsorbed dirt in sunlight.
In 2001, Pilkington Glass (Pilkington) announced the development of the first self-cleaning windows, Pilkington Activ, soon after PPG.LTD (PPG) released similar product of PPG’ Sunclean. All of these windows are coated with a thin transparent layer of titanium dioxide (titania or TiO2), a coating which acts to clean the window in sunlight through two distinct properties: photocatalysis causes the coating to chemically break down organic dirt adsorbed onto the window, while hydrophilicity causes water to form ‘sheets’ rather than droplets – contact angles are reduced to very low values in sunlight (the coating becomes ‘superhydrophilic’), and dirt is washed away.
TiO2 has become the material of choice for self-cleaning windows, and hydrophilic self-cleaning surfaces because of its favorable physical and chemical properties. Not only is highly efficient at photocatalysing dirt in sunlight and reaching the superhydrophilic state, it is also non-toxic, chemically inert in the absence of light, inexpensive, relatively easy to handle and deposit into thin films and is an established household chemical – TiO2 it is used as a pigment in cosmetics and paint and as a food additive (Macwan et al).
Despite the commercialization of a hydrophilic self-cleaning coating in a number of products (Pilkington glass and PPG’s Sunclean), the field is far from mature; the development of new materials that can be easily applied on facades, with both self-cleaning and de-polluting properties would be a significant step towards improvements in urban quality of cities. Investigations into the fundamental mechanisms of self-cleaning and characterisations of new coatings are regularly published in the primary literature.
A thorough discussion of the theory of photocatalysis and super-hydrophilicity is beyond the scope of this study, hence only a brief summary of photocatalysis follows. Greater detail can be found in one of several review articles (Fujishima et al 2008).
“Schematic drawing of self-cleaning process on TiO2 nano coating”
Phase Change Materials
(PCMs) can be described as mixtures of chemicals having freezing and melting points above or below the water freezing temperature of 0°C (32°F). PCM materials are ideal products for thermal management solutions as they store and release thermal energy during the process of melting & freezing (changing from one phase to another).
When such a material freezes, it releases large amounts of energy in the form of latent heat of fusion, or energy of crystallisation. Conversely, when the material is melted, an equal amount of energy is absorbed from the immediate environment as it changes from solid to liquid.
To be a useful PCM, a material has to meet several criteria:
- Release and absorb large amounts of energy when freezing and melting; This requires the PCM to have a large latent heat of fusion and to be as dense as possible.
- Have a fixed and clearly determined phase change temperature (freeze/melt point); The PCM needs to freeze and melt cleanly over as small a temperature range as possible. Water is ideal in this respect, since it freezes and melts at exactly 0°C (32°F). However, many PCMs freeze or melt over a range of several degrees, and will often have a melting point that is slightly higher or lower than the freezing point. This phenomenon is known as hysteresis.
- Avoid excessive supercooling; Supercooling is observed with many eutectic solutions and salt hydrates. The PCM in its liquid state can be cooled below its freezing point whilst remaining a liquid. Some salt hydrates can be cooled to +50°C (122°F) below their freezing point without crystallisation occurring. This can be beneficial, for example in hot packs where a +48°C (118.4°F) PCM is kept as a supercooled liquid at room temperature until the hot pack is required and supercooling is broken by mechanical or chemical nucleation. However, for most applications, supercooling must be kept to a minimum by the addition of suitable nucleating agents to the PCM.
- Non-hazardous; PCMs are often used in applications whereby they could come in contact with people, for example in food cooling or heating applications, or in building temperature maintenance. For this reason they should be safe. Ideally a PCM should be non-toxic, non-corrosive, non-hazardous and non-flammable. There are many substances that behave excellently as PCMs but cannot be used due to issues over safety.
- Remain stable and unchanged over many freeze/melt cycles; PCMs are usually used many times over, and often have an operational lifespan of many years in which they will be subjected to thousands of freeze/melt cycles. It is very important that the PCM is not prone to chemical or physical degradation over time which will affect the energy storage capability of the PCM.
Some eutectic solutions may be susceptible to microbiological attack, so must be protected with biocides. Long term stability can be a problem in some salt hydrate PCMs, unless they are modified to prevent separation of the component materials over successive freeze/melt cycles.
- Economical; It doesn’t matter how well a substance can perform as a PCM if it is prohibitively expensive. PCMs can range in price from very cheap (e.g. water) to very expensive (e.g. pure linear hydrocarbons). If cost outweighs the benefits obtained using the PCM, its use will be very limited.
The simplest, cheapest, and most effective phase change material is water/ice. Unfortunately, the freezing temperature of water is fixed at 0°C (32°F) making it unsuitable for the majority of energy storage applications. Therefore a number of different materials have been identified and developed to offer products that freeze and melt like water/ice, but at temperatures from the cryogenic range to several hundred degrees centigrade.
PCMs can broadly be arranged into five categories: eutectics, salt hydrates, organic, solid-solid and molten salt materials.
- Eutectics tend to be solutions of salts desolved in water that have a phase change temperature below 0°C (32°F).
- Salt hydrates are specific salts that are able to incorporate water of crystallisation during their freezing process and tend to change phase above 0°C (32°F).
- Organic materials used as PCMs tend to be polymers with long chain molecules composed primarily of carbon and hydrogen. They tend to exhibit high orders of crystallinity when freezing and mostly change phase above or below 0°C (32°F). Examples of materials used as positive temperature organic PCMs include alcohols, waxes, oils, fatty acids and polyglycols.
- Solid-Solid PCMs that undergo a solid/solid phase transition with the associated absorption and release of large amounts of heat. These materials change their crystalline structure from one lattice configuration to another at a fixed and well-defined temperature, and the transformation can involve latent heats comparable to the most effective solid/liquid PCMs.
Such materials are useful because, unlike solid/liquid PCMs, they do not require nucleation to prevent supercooling. Additionally, because it is a solid/solid phase change, there is no visible change in the appearance of the PCM (other than a slight expansion/contraction), and there are no problems associated with handling liquids, i.e. containment, potential leakage, etc.
- Molten Salts are naturally solid salt materials which turn liquid when they are heated above their transition temperatures and act as a PCM energy storage material.
Clathrates may also be used. They look like a solid crystalline material, store large amounts of heat when they melt, and some have melting temperatures that are attractive to energy storage applications. Unfortunately, clathrates can currently only be produced by mixing the the host and trapped species under very large pressure, and once the clathrate has melted the two species cannot easily be reincorporated, thus can only be used once as a PCM.
Some liquid/gas phase change materials can be used the energy storage but they tend to involve large changes in volume or pressure when going from the liquid to the gaseous phase, this prevents effective and economical encapsulation.
A large number of PCMs that melt and solidify at a wide range of temperature exist and are used in many applications. Energy storage is a significant alternative for exploring new source of energy. It not only reduces the mismatch between supply and demand but also improves the performance and reliability of energy systems. Saving in premium fuel could also be achieved by reducing the wastage of energy and capital cost. For example, the performance of building cooling and heating system could be improved by storing the thermal energy for a later use. However, in the large scale system that employs PCM storage system, the correlation between charging and discharging performance and the system factors (coolant flow velocity and temperature) is yet to be well understood so as to determine the design and construction of PCM storage system.
In order to develop the model that is to be used within the HERB project, both active and passive systems should be taken into account. For example, integration into the façade technology would be considered active as air flow would be directed over an area containing the material; in a passive system, such as incorporation onto the surface of solid wall insulation, the application would be considered passive. This report demonstrates how the model developed for the HERB project was adapted for use in an existing technology.
“ Commercially available PCM solutions”
Aerogel and Vacuum Insulated Panels (VIPs)
Currently two types of super insulation are commercially available: Aerogel and Vacuum Insulated Panels (VIPs).
Aerogel materials exhibit the lowest thermal conductivities of any of the solid or porous materials. The thermal conductivity of aerogels is around 0.014 Wm-1K-1, and is about 100 times smaller than that of full density silica glass. It is derived by removing the liquid component of the material with a gas. This process is known as supercritical drying. If the gel were allowed to dry normally the capillary action, of the liquid being extracted, would cause the solid to collapse. When supercritical drying is used this does not occur.
Other aerogels have been developed including those based on: alumina, chromia, tin oxide and carbon. In physical appearance aerogels are solid, rigid, and dry material, their name comes from the fact they are derived from gels. The physical content the material is over 99.99% air and this provides it with very favourable insulation properties making it useful for both buildings and apparel. In addition to insulation aerogel has extremely high hygroscopic properties. This makes it a very strong desiccant, and this property has led to its use by NASA in absorbing comet dust in outer space. However, in building applications it is considered to be a hindrance as the blanket format, which it is supplied in, is difficult for installers to work with. This is because when touched with bare hands, the blanket removes moisture from skin as it is a dessicant; additionally nano-sized dust is released into the air, which acts as an irritant when inhaled. In a study undertaken in the University of Nottingham, where aerogel was installed into one room of a solid wall house, aerogel dust was detected on the surfaces of worktops and furniture months after install. Currently there is only one supplier of Aerogel Products within the UK, the Proctor Group.
Vacuum Insulated Panels are an innovative technology in which a rigid fumed silica core material is wrapped in a sealed air tight material (such as aluminium) from which the air has been evacuated. With the air removed from the material its insulation properties are five to ten times greater than conventional insulation. The coating film is composed by plastic polymers (Polyethylene Terephthalate [PET], Polyethylene [PE], Polypropylene [PP]) alternated with metal layers, (Al, Steel, AlOx, SiOx) that prevent gaseous penetration inside the core. A section of a film envelope is displayed.
VIP present the lowest values for thermal conductivity between the insulating materials currently available on the market, at the centre-of-panel the thermal conductivity can be as low as 0.004Wm-1K-1. In respect to this the thickness of the material can be very low to achieve the required U-value. VIPs have been used for many decades in appliances including fridges and freezers; it is only recently that they have been proposed for use in buildings.
“Manufactured aerogel sample and in thermal insulation effectiveness”
Passive zenithal light guides
Passive zenithal light guides, are a type of Tubular Daylight Guidance Systems, which are daylight systems that can chanel daylight to the core of buildings. Passive zenithal light guides, or more commonly light pipes, comprise of three types of elements: 1. Light collector, which is usually a glass dome of high transmittance value, 2. a means of light transport, which is an alouminioum tube with reflectance value greater than 0.95 and 3. An element which distributes daylight into the space, usually an opal or prismatic diffuser. Light pipes can be horizontal or vertical and they can even have bends, in order to suit any application.
During the last few years, light pipe manufacturers have tried to incorporate artificial lighting into traditional light pipes, so as the produced system can provide the desired interior illuminance levels under any sky conditions. In basic versions, the artificial lighting is provided by spotlights (either halogen or LED lamps), supported by a small arm, located relatively close to the diffuser (about 0.20-0.5m above the diffuser inside the tube). The more sophisticated versions incorporate LED lamps, with specially designed optical systems for the maximization of the system performance, combined with daylight linked contols, in order to achieve great energy savings.
This report describes the methodology that was followed, for developing a reliable computer model for the calculation of the interior lighting conditions created by a light pipe with integrated LEDs and for the assessment of the energy performance of this system. The methodology includes:
Application of some calculation-simulation methodologies, for various spaces and light pipe configurations.
More specifically, the theoretical algorithms that are used for the calculations, are: 1. The method analyzed in the CIE Technical Report 173:2006 “Tubular Daylight Guidance Systems”, refered to as the TTE method and 2. the Luxplot method, developed by Jenkins and Muneer (2004).
The simulation methodologies, with the use of the IES VE pro software, include: 1. The realistic modeling of the light pipe and also the modeling of the system with a number of assumptions about the dimensions and the element transmittances, 2. The modeling of the light pipe as a luminaire of cosine luminous intensity distribution
- Development of two algorithms for the calculation of the light pipe performance.
- Development of a methodology for assessing the number and/or wattage of the LEDs needed to supplement the natural light in a space.
- Assessment of the energy savings of the system.
- Examples of the application of the developed algorithms and methodologies.
PV Systems-Façade Integrated Pv Systems
SUAS developed an improved PV-model and extended the model by the introduction of further application fields. Whereas the basic model covers free standing PV-fields with ambient temperature on the surface and on the backside of the panels regarded to be the same, the extended PV model also covers façade integrated PV-systems with ambient wind speed at the surface, air flow at the backside either generated by natural or forced convection and considering backside temperatures differing from ambient temperature. The PV-models were implemented in analogy to the thermal systems in the overall energy supply interface described above and connected to electricity demand results from EnergyPlus for a given building in order to e.g. define the ratio between self-use of produced electricity and electricity fed to the grid.As an example for PV-system integration and performance analysis, a model of a free standing PV-system as well as an inverter model were set up and implemented in Insel. Calculations were carried out at the example building in Almada, Portugal.
“ Wiring diagram for one PV-field”
Solar Thermal Heating Systems
SUAS developed a number of mathematical models for heat supply for buildings, such as thermal solar collector, biomass furnace, stratified heat store and heat sink and combined the other ways isolated models in a system integration interface. The implementation was done in Insel, a simulation platform developed at the University of Applied Science-Stuttgart, SUAS. In order to make the mathematical models available to the other HERB-Partners these models are also implemented in MatLab-Simulink for integration in other simulation Software such as e.g. TRNSYS.
Among the technologies for energy generation from biomass, combustion is the most advanced and market proven application. The process of biomass combustion consists of a number of individual and interlinking steps of high complexity. The operation of a combustion appliance depends both on the fuel properties and furnace control parameters. Even though the combustion of solid biomass is extremely complex, there are a number of engineering equations which can be solved relatively easily. These calculations are based on mass balances and thermodynamics and were implemented in the mathematical model for biomass combustion. The equations used in the simulation describe the overall fuel conversion reaction taking place and the thermal performance.
The model input parameters include the properties of water (mass flow, feed and return temperature) which has to be heated during combustion. Important simulation parameters for the biomass combustion appliance are fuel properties, system performance (nominal and minimal output performance) and the amount of emitted pollutants. The standard combustion system parameters can be found in data sheets given by manufacturers of the heating appliances.
The combustion system converts the chemical energy of the fuel to internal energy of the gases within the combustion appliance. The heat of combustion is defined as the enthalpy difference before and after combustion.
“Heating system chart involving solar thermal appliance and auxillary heating.”
“Heat supply with a biomass burning system.”
Air to Water Heat Pump
In order to reduce the fossil fuel consumption, heat pumps are becoming an important technology for heating and cooling buildings and for domestic hot water production. In particular, air-to-water heat pumps are widely used for heating residential buildings.Several researches aimed to analyze and to enhance the performance of air-to-water heat pump systems are available in the literature. Relevant researches regarded, for instance, the optimization of the evaporator and defrost cycles, the effects of the water supply temperature on the seasonal COP, the choice of the working fluid and the optimization of the refrigeration cycle , the use of gray water as heat source, the increase of seasonal COP obtainable by employing heat pump systems with inverter and a variable speed compressor, with respect to on-off heat pump systems.A topic not yet sufficiently investigated in the literature is the optimal choice of the balance point and of the thermal storage (puffer) volume for heating plants with electric air-to-water heat pumps. The value of the storage volume recommended by several manufacturers is, in litres, 60 times the heat pump nominal thermal power, in kilowatts. The European standard EN 14825 indicates a bivalent temperature (or, balance-point temperature) of – 7 °C or lower for the reference heating season C (colder climate), 2 °C or lower for the reference heating season A (average climate), and 7 °C or lower for the reference heating season W (warmer climate). Systematic studies on this subject, with reference to several climates and heat pump types, are not available.To ensure the optimal design of air-to-water heat pump systems to be applied in the energy retrofit of the HERB buildings, a numerical code to determine the optimal balance-point temperature and thermal storage volume of air-to-water heat pump heating systems has been implemented through the software MATLAB. The code can be used for hourly simulation and optimization of both heat pump systems with an inverter compressor and on-off heat pump systems. Examples of employment of the code to determine the optimal balance point and the optimal storage volume have been developed, with reference to the climate of Bologna (North-Center Italy).