2.6 HEAT RECOVERY, ENERGY STORAGE AND ACTIVE SOLAR SYSTEMS

Transcription

1 2.6 HEAT RECOVERY, ENERGY STORAGE AND ACTIVE SOLAR SYSTEMS Integration of Heat Recovery, Energy Storage and Active Solar Systems for Improvement of Indoor Air Quality, Energy Efficiency and Optimisation of Operation Costs A general problem of museums are the contrasting conservation requirements of exhibition objects and the comfort requirements of the museum visitors. This diverging requirements usually lead to a complication of the definition and the improvement of the indoor air quality and thermal comfort. Further on historical constructions often make it difficult to integrate building services like air channel systems or active solar. The demands on the preservation or protection of these monuments restrict the variety of solutions. For this the integral planing and the inclusion of experts as early as possible is an important aspect for the design or retrofitting concept of energy efficient museum buildings. Heat recovery, energy storage, and active solar systems are suitable means to achieve energy efficient buildings and to reduce their operating costs. In detail it means to reduce the annual energy demand for heating and cooling, the electricity consumption and, in consequence, the CO 2 -emission of buildings. The integration of heat recovery systems or components leads to a reduction of thermal ventilation losses of the building and to a reduction of the required energy support. Energy storage can be inserted to transfer excessive gained or recovered energy from a period of time with surplus energy gain to a period of time when this surplus energy can be used as supply again. The use of internal energy storage resources in walls and ceilings can enable an improvement of the indoor air climate and the thermal comfort with relative low additional costs. The use of renewable energy like active solar systems leads to a conservation of natural resources in addition. Systems like solar air and water collectors usually have to be combined with energy storage systems to shift the energy to a later period of time when it is needed. Photovoltaic elements also lead to a reduction of primary energy consumption and CO 2 -emission. The integration of existing structural elements like walls and ceilings inside the building and its foundation as energy storage components and the implementation of heat recovery systems and active solar components within an energy design can lead to economically and ecologically optimised solutions also for museum buildings. The dimension of the different components have to be co-ordinated to reach advanced solutions. Due to the complex requirements on indoor climate and monument conservation the concepts have to be custom-made.

2 State of the Art Heat Recovery, Energy Storage and Active Solar Systems Design Precondition to reach energy efficient buildings is the reduction of thermal losses. The improvement of the thermal insulation and the airtightness of the building in combination with a controlled air change decrease the required supply energy. This offers the use of energy optimised low temperature heating and high temperature cooling systems. Heat recovery, energy storage and the integration of active solar systems can be inserted as basic components of advanced energy concepts. In practice different system of heat recovery, energy storage and active solar are possible. In case of already existing ventilation systems the integration of heat recovery is a sensible measure for saving energy and operation costs. Air to Air, Air to Water and central or decentral systems are different possibilities to realise these systems. Central air to air systems are usable in new and existing museums buildings with already integrated air channel systems. The exhaust and supply air have to be placed central in close connection. Standard heat recovery components are recuperators or regenerators. Decentral air to air systems are particularly suitable for retrofitting existing museum buildings. It is possible to add fresh air to circulating air. In this way the air change can be minimised to the smallest amount required by hygienically aspects. The air conditioning can be realised as a four-wire-system with cooling and heating coils. Generally air to water systems are less efficient than air to air systems, but central air to water systems are favourable in case of distant adjustment of supply and exhaust air channels or in case of simple exhaust air systems. In the second case the recovered heat can support low temperature systems like floor and wall heating. The thermal use of construction units or natural storage capacities in the environment offers another way to increase the energy efficiency of a new or an existing building. Most simple is the activation of thermal masses like ceilings, floors and walls by ventilation. In consideration of security aspects a nocturnal ventilation exhausts the heat stored over the day. In the morning pre-cooled construction units in combination with a reduced air change stabilise the indoor climate and avoid an overheating of the rooms. The system efficiency depends on the capacity of the construction material. Especially old museum buildings with big construction masses are suitable. The advantages of this solution are the simple system configuration and in consequence low capital costs and low energy consumption. In new buildings concrete core activation is a possibility for using construction units for energy storage and active cooling or heating. Therefore water filled tubes are integrated for example in the ceiling construction. Central air to air system Decentral air to air system Central air to water system Activation of thermal masses by ventilation Concrete core activation and heat exchanger piles

3 Energy piles and borehole heat exchangers use the surrounding ground for heat exchange and thermal energy storage. Heat exchanger piles are additionally used to support the building foundation. Both systems work with plastic tubes, in which a fluid circulates to exchange heat with the environment. In most cases the heat carrier fluid is water or a water-glycol mixture. An advantage is the parallel usage of foundation piles as channel and foundation system. The extra costs for construction are reduced to the integration of the tubes in the reinforcement basket and the connection of the different vertical U-tubes arranged in the piles. Generating energy with active solar systems is a possibility of using renewable energy. Solar assisted space heating, solar tap water heating, solar air heating and solar cooling are systems using thermal solar energy. The efficiency of a solar assisted heating system depends on the thermal insulation of the building. Suitable are buildings of low-energy-standard (heat demand up to 70 kwh/(m²a)) in connection with low temperature heat distribution systems like floor or wall heating. Due to the missing heat demand in summer, solar assisted space heating normally is not economic for museums. In case of further heat demand or the addition of further energy consumption units within an district heating system the integration of a solar system has to be considered. In this case the roof of the museum can be used for the arrangement of the collector area (if a greater coherent roof area is available). Economic solar tap water systems start by a daily hot water demand of 500 l. In case of low hot water demand and noncentral tapping points as in most museum buildings the integration of a plant for the thermal use of solar energy is not recommendable. For this decentral electric flow heaters should be installed to reduce investment costs and heat losses (standby and distribution losses). Solar air collectors transfer energy directly to the air flow inside the collector. In an open loop system the pre-heated air can be used as supply air for building ventilation. Another possibility is the integration of the solar air collector in a closed loop. In this case the circulating air is used as heat carrier for energy transfer to the building structure, like a hypocaust system. Solar cooling systems use the direct coupling of the cooling demand and the available solar energy both rise simultaneously. Heat delivered by a solar collector generates warm air to dehumidify and to cool down a water absorbing material (evaporative cooling). This cooling energy is transferred to the air of the ventilation system. Opposed to solar heating absolutely no storage is needed. In addition to the thermal use of solar energy, photovoltaic systems transform solar energy into electrical energy. The electrical energy can be used inside the building for every purposes or it can be fed into the grid. The possibility of the fabrication of PV-elements in different forms, colours and sizes offers a wide range of possibilities for their architectural integration. A double function as shading device and energy collector leads also to a reduction of the additional investment costs for the integration of PV-elements. Solar tap water system Solar air collector, open loop Solar air collector, closed loop Solar cooling system Photovoltaic integrated to a shading system

4 Heat Recovery, Energy Storage and Active Solar Systems Design Techniques to Optimise Performance In case of the improvement of the indoor air quality and thermal comfort different conditions have to be complied. Climate demands of people and exhibits have to be defined and differentiate by agreeing and dissenting requirements. In the second case sealed spaces and/or showcases for the exhibition parts have to be implemented. Demands on the preservation or protection of the outward appearance of the building, regarding historical monuments, have a great influence on selection and design of components of energy concepts for museums and restrict the variety of solutions. Therefore a sensible combination of system components is necessary. Hybrid systems should be inserted in an integral concept. For this the integral planing and the inclusion of experts as early as possible is an important aspect for the design or retrofitting concept of energy efficient museum buildings. In the following some expedient applications and combinations of components are composed. The integration of heat recovery and therefore the reduction of ventilation losses leads to a decrease of the heat demand of the museum building and offers the possibility of the integration of low temperature heating and high temperature cooling devices. A controlled air change and the airtightness of the building envelope are preconditions of the installation of heat recovery units. Standard air to air heat recovery components are recuperators and regenerators. The regenerator can be used for heat exchange as well as for humidification and dehumidification of the supply air. The heat exchange between the two air flows in regenerators is realised by alternative passing of the heat exchanger by the two flows. Driven by the partial pressure gradient between in- and outgoing air also a moisture exchange between the two flows is possible. In the recuperator the two air flows are totally separated. The exchange of moisture is not possible. To avoid condensation and freezing in the recuperator (exhaust air) the outside air has to be pre-heated during frost periods in winter. An advantage of central heat recovery systems is the simple integration of components like other heat sources and heat pumps. Also an earth heat exchanger for pre-heating or pre-cooling could be provided. In summer the earth heat exchanger achieves K lower temperatures of the supply air than outside air for a period of time. In winter the earth heat exchanger avoids condensation water and freezing in the recuperator. The disadvantage of the combination with an earth heat exchanger is the less effectiveness of the recuperator because of the small temperature gradient between the two air flows. Recuperator Regenerator

5 The thermal activation of ceilings by embedded coils for example is an useful combination of an active component for heating / cooling with an additional energy storage function. For concrete core activation the medium temperature is approximately 20 C (corresponding to the required indoor temperature). In a self regulating effect which depends on the temperature gradient between water and indoor air the ceiling is used for cooling or heating. As heat source for concrete core activation often the environment in connection with energy piles is used. In summer the cold of the ground is used for direct cooling. Coupled with a heat pump, which generates a higher temperature range, the system can be used for heating in winter. Generally, the heating power of concrete core activation does not cover the heat demand of a building. Therefore the system is normally supported by a conventional space heating system. Energy storage components outside the building like energy piles are possible systems to shift energy amounts over a defined time period or to use earth heat. Ceiling with a system of plastic tubes for concrete core activation The implementation of active solar systems is independent of the age of the building. For historic buildings demands on the preservation or protection of historic monuments have to be regarded. Integral planning of new buildings can lead to solutions which are able to fit multiple functions. A photovoltaic facade for example generates power and protects the building against weather conditions (rain, wind, solar irradiation). Therefore, the first step in designing active solar systems is to check possible solar receiver surfaces (roof and facade) and the demand of solar energy (solar heating / cooling or electric power). Suitable are collector arrangements with tilt angles of and vertical elements with orientations from south-west to south-east. The use of active solar components for the energy supply in museums usually is restricted to the integration of photovoltaic elements, solar air collectors and solar cooling. In special cases the thermal use of solar energy can be utilised for heating and tap water preparation. Reinforcement and U-tubes of energy piles The integration of solar water collectors depends on the heat and first of all on the hot water demand and is not common for museums. However, in some cases simple systems with absorbers are suitable. These systems are generally used for swimming pools, but they are also suitable in connection with low temperature systems like wall tempering. For this the wall is used as storage. Flat plate and vacuum collectors are used for higher temperature ranges like tap water or space heating. Vacuum collectors are more energy efficient than flat plat collectors and can be easily oriented into the required slope, but they are also more expensive. By integration of the corresponding control and hydraulic parts the collector of a solar cooling system can be used for cooling in summer as well as for heating in winter.

6 Other sensible applications for the thermal use of active solar components within energy concepts for museums are solar air collectors, which pre-heat the air before entering the building. The absorbed radiation energy is transferred directly to the air flowing through. In an open loop system the air can be used as supply air for building ventilation. These systems are an optimal addition to low temperature systems and systems with a high thermal inertia. In case of solar air collectors heat recovery is not necessary. Another possibility is the integration of the solar air collector in a closed loop. Here the circulating air is used as heat carrier for energy transfer to the building structure, like a hypocaust system. The structure warms the enclosed spaces by radiant heat. In this case a powerful and speedy heating systems is needed in addition. The low density and heat capacity of air compared to water as heat carrier cause large channel dimensions and volumes, which have to be considered in the building design. Solar air collectors integrated to the facade or the roof of a building in addition act as building insulation. The simple, lightweight and cost-efficient construction of the solar air collector are other advantages. As already described, different solutions of integrating photovoltaic elements are possible. Hybrid systems like photovoltaic facades, which generate power and protect buildings against weather conditions, roof sealing elements with implemented photovoltaic cells or photovoltaic cells integrated in a shading system are available on market. Nowadays Photovoltaic elements occur in different kinds of colours and geometry. Also partly translucent elements are possible. Therefore the integration of photovoltaic could be an element of architectural and lightning design. Because of extensive coherence of building and system engineering and numerous dynamic boundary conditions like temperature and moisture, the design and optimisation of energy systems has to carry out with computer simulations. Different tools allow the investigation of system behaviour at different load cases and offers the opportunity to optimise the system configuration. The coupled dynamic building and system simulation enables the accommodation of different components on each other and on the building demand. The simulation results give design certainty and are an important tool for the development of efficient and adapted concepts for the complex demands on indoor air quality and thermal comfort in museums. Hybrid system shading system with photovoltaic Building surface of translucent photovoltaic modules Photovoltaic elements of different colours and geometry

7 Heat Recovery, Energy Storage and Active Solar Systems Design Performance Criteria In energy optimised buildings the ventilation losses can amount to over 50% of the thermal losses of the building. To reduce these losses and to reach a low energy standard for the building (net heat demand lower than 60 to 50 kwh/(m²a)) the integration of a heat recovery unit is required. Central air to air heat recovery devices in any size and construction (recuperators, regenerators, heat pumps etc.) can be integrated into a ventilation system for supply and exhaust air. Normally a sensible energy recovery rate of 70 to 90 % can be expected by using recuperators (channel counter flow units). Regenerators used as enthalpy wheels can also recover humidity and reach humidity recovery levels of 50 to 60 %. In any case the COP of heat pumps should be greater than 3 to save primary energy. The efficiency of the thermal use of construction units depends on the capacity and on the thickness of the construction material (see picture). Low capacity and/or thin constructions like light weight constructions can be covered with a plaster including a phase change material. The plaster stores energy at a specific temperature or temperature range without temperature rise until all the phase change has happened. This effect is adjustable by the choice of material in a range between 23 to 26 C. The latent heat is circa 110 J/kg. Therefore phase change materials can make light weight constructions behave thermally like heavy weight constructions. According to the aim of cooling or heating the water temperature range for concrete core activation is about 18 to 28 C. As already described water temperatures corresponding to the required indoor temperature achieve a self regulating effect. The activated building unit cools or heats depending on the temperature gradient between water and indoor air. In order to adjust water and indoor temperature the temperature spread between outflow and inflow has to be 2 to 5 K. Apart from the temperature gradient and the water flow the cooling or heating power depends on the position and distance of the tubes in the building unit. In case of an activated ceiling the tubes could be placed at the top, at the bottom or in the core. The distance of the tubes amounts generally 15 to 30 cm. In case of the mentioned boundary conditions the cooling or heating power achieves a value of 5 to 40 W/m². Energy piles and borehole heat exchangers exploit the surrounding earth as storage medium for heat and cold. In case of energy piles the number and depth of the piles usually depend on building constructional necessities. In case of separate piles only for energy transfer purposes important influences of the distance between the piles (greater than 5 to 6 m), their depth (usual range 30 to 150 m) and of course the relevant parameters of the ground (thermal conduction, ground water flow, humidity) have to be regarded within their dimensioning. The diameter of the tubes used for the circulation of the fluid is usually about 26 to 46 mm. The dimensioning of the tubes depends on the depth, the set up of the Influence of material and material thickness on the storage efficiency water ~20 C C indoor temperature 18 C 22 C heating cooling Effects of self regulating for thermal activated constructions 28 cm 28 cm heating power heating power tube distance 15 cm 25 W/m² 10 W/m² tube distance 30 cm 12 W/m² 15 W/m² cooling power cooling power 13 W/m² 15 W/m² 6 W/m² 22 W/m² Cooling and heating power depending on the tube position and distance (steady-state condition log T = 5 K), IndustrieBAU 2/99

8 piles, the corresponding heat transfer rate and the resulting pressure drop. Depending on the ground parameters a medium heating or cooling power in the range of 50 to 70 W/m can be expected. To be sure of the long time development of the temperature level within the ground simulation studies have to be carried out. Earth heat exchangers can be used for the pre-heating or -cooling of fresh air before entering the ventilation unit or the building. The main influences on the reachable heating or cooling power of an earth heat exchanger are parameters of the earth (thermal capacity, humidity...), its disposition (under or beside the building, depth) and the length of the channels. To reach a good efficiency the depth of the earth heat exchanger should be chosen to 1,8 to 2,4 m (6 to 8 ft). The channels have to be positioned in the beginning frost free zone of the ground therefore the depends on climate conditions. To reach a good heat or cold transfer within the earth heat exchangers and to avoid unwanted high pressure losses the diameter of the channels should be adapted to the mainstream. The corresponding flow velocities should be about 1,5 to 3 m/s (300 to 600 fpm). With a length of about 35 to 45 m a temperature level of 0 C can be reached at the end of the earth heat exchanger at ambient temperatures of about -15 C. In summer the air can be cooled down to a temperature level of below 20 C at ambient temperature of 25 to 30 C. For active solar systems collector arrangements with tilt angles of and vertical elements with orientations from south-west to south-east are suitable. In addition to the tilt angle and the orientation of the collector the yearly energy gain depends on the yearly global irradiation and on the size of the collector area. Further on shading of close-by buildings, trees as well as other collectors have to be considered when planning solar receiver surfaces. Solar tap water systems can work economically only from a daily hot water demand of about 500 l. Therefor in most cases only museums with cafeterias or restaurants are recommendable. To achieve a yearly solar fraction of about 50 % 1 m² collector area per 50 l daily hot water demand (water temperature 45 C) have to be installed. The store volume should be 60 l per square meter. The heat gain of solar air collector systems depend on numerous factors like the yearly global irradiation, the dimension of the collector area and on the type, the utilisation and the air change of the building. The efficiency of solar air collectors increases with low temperature gradients between outflow and ambient air and with a rising airflow. In general solar air collector systems are able to generate 100 to 350 kwh heat per year and square meter collector area for heating and ventilation. The efficiency of photovoltaic depends on the kind of installed solar cells (amorphous silicon, polycrystalline silicon or monocrystalline silicon cells). The ratio between generated power and solar radiation lies in a range of 5 to 18 %. Therefore the efficiency of the solar cells the orientation, the tilt angle and the yearly global irradiation are the main influences on the electrical energy gain. preheating of air in winter: temperature [ C] length of pipe [m] ϑe.. earth temperature ϑ1.. temperature of air in the pipe at ϑa= -15 C ϑ.. temperature of air in the pipe at ϑ = -20 C cooling of air in summer: ϑ E ϑ 1 ϑ 2 2 A temperature [ C] ϑ 1 ϑ 2 ϑ E length of pipe [m] ϑe.. earth temperature ϑ1.. temperature of air in the pipe at ϑa= 30 C ϑ.. temperature of air in the pipe at ϑ = 25 C 2 A parameters: V air=155m³/h; depth of laying 1,8m; length of pipe 40m; Reachable air outlet temperature in earth heat exchangers tilt angle south tilt angle east, west Influence of the tilt angle of the solar collector to the received solar irradiation

9 Results and Lessons learned with respect to the Heat Recovery, Energy Storage and Active Solar Systems Design Energy storage, heat recovery and the use of active solar systems can be integrated in energy design concepts for museum buildings and contribute to lower the energy consumption and to reach a low energy standard. The knowledge of the operation characteristic of the different measures is important for reasonable system integration and the development of efficient concepts. The integration of energy storage devices like energy piles or even construction units within the thermal activation of ceilings improve energy consumption and thermal comfort. The use of active solar components for the energy support of museums usually is reduced to solar air collectors or photovoltaic. Heat recovery units can be inserted as decentral or central systems. Different already known solutions can be applied and contribute to reduce the energy consumption. The special requirements of the exhibition rooms and the problem of the divergence of the indoor air requirements for exhibits and visitors within the museum has to be regarded in any case. Coupled tools for system and building simulation should be used to predict the parameters of thermal comfort and indoor air quality and to adjust the different components of efficient energy concepts. Within the museum project several advanced energy designs using active solar components, heat storage and ventilation systems were realised and monitored. A very good energy concept including solar air collectors energy storage and heat recovery has been implemented at the Museum of Modern Art at Kristinehamn, Sweden. The museum is hosted in a former hospital which was build during the 1960s and was rebuild to an art gallery in In the meantime the gallery needed more space. Therefore a nearby, former boiler house was rebuild and incorporated. Because of the insufficient thermal insulation new economic and innovative methods for heating were necessary in order to use the building in wintertime. The solution is a south orientated air collector with an area of 48 m² which has been mounted at the former charcoal tower for pre-heating ventilation air in winter. Outside air flows through the collector. The solar heated air is driven to the bottom of the tower and through the basement by fan. Solar energy is stored by the thermal mass of the building structure. Afterwards the tempered air is used for ventilation of the exhibition halls. The exhaust air is evacuated through the tall charcoal tower (See principle scheme). When the museum is closed the exhibition halls are ventilated by recirculating air. In summer seawater is used for cooling the supply air. Secondary rooms like offices, library, cafeteria and shop are ventilated conventionally by a ventilation system with a central air to air heat recovery. The main energy supply of the floor heating in the exhibition halls and the heating in the secondary rooms as well as the post-heating of the ventilation air is provided by district heating. Museum of Modern Art with the charcoal tower at Kristinehamn Solar air collector installed to the charcoal tower

10 Principle scheme of the solar air heating at the Museum of Modern Art at Kristinehamn The C/PLEX Art Centre at West Bromwich in Great Britain shows, that planning energy efficient buildings already starts with the building design. In this example a special heat recovery mechanism is integrated to the building design. Therefor a concept with different climatic zones is realised. At the moment the C/PLEX Art Centre is under construction. It will be a modern, flagship building containing gallery-, work- and creative spaces, retail opportunities, restaurant facilities, public areas, conference rooms and other multi-function spaces. Due to the diverse nature of the spaces, a number of distinct comfort requirements exist. These can be broadly distinguished into three groups: - Areas requiring close control like exhibition type spaces. - Areas with more relaxed comfort requirements, where natural ventilation and heating is proposed like office and circulating spaces. - The bioclimatic area like general public areas and some exhibition spaces, closely following outside conditions, where minimal heating is proposed. The bioclimatic areas are located within the bioclimatic enclosure which takes the form of an intelligent building skin. The close control areas lie entirely within the bioclimatic enclosure and therefore do not experience the outside climate but the mitigated conditions created by the bioclimatic enclosure. A heat recovery mechanism is specifically based upon the box within a box strategy. If appropriate exhaust air from the closed conditioned spaces, is dumped into the bioclimatic zone. For pre-heating and pre-cooling the close controlled spaces and some of the naturally ventilated spaces borehole heat exchangers will be installed at the C/PLEX Art Centre. Box within a box strategy of the different climate zones