Heating and Cooling with Advanced Low Exergy Systems


Abstract

There is an obvious and indisputable need for an increase in energy efficiency in buildings. Until now, so called “energy savings measures” and analyses of energy flows in buildings have commonly been based on the energy conservation principle, the first law of thermodynamics. An exergy analysis which combines both the first and second law of thermodynamics allows a better understanding and more effective design of energy flows in buildings. That way a more holistic view on the calculations is gained for all processes involved.

In accordance to these findings, new innovative heating, cooling and ventilation appliances have been developed within the framework of a German national collaboration project. This short introduction describes both the structure and work plan of the German alliance project on low exergy systems and the achievements in the development of innovative systems brought about by this close collaboration between leading German industry and research institutes.

Keywords

Exergy analysis, energy efficient buildings, innovative HVAC systems

Introduction

In Germany, as in other industrialised countries, about 40% of the total energy consumption is induced by the heating, cooling and ventilation demands of buildings (ECBCS 2006). To keep indoor environments at a comfortable temperature of about 20°C throughout the year, mostly fossil energy sources of high quality, e.g. fuel oil, natural gas or electricity, are used. For economic and ecological reasons it is necessary to increase energy efficiency to a maximum extent and to use energy as effectively as possible.

Recent pilots show that the heating and cooling demands of rooms can be supplied using systems other than traditional heating boilers and fossil energy carriers. New and innovative systems use very small temperature differences between the heat carrier medium and the room. This way high quality energy sources can be used more efficiently and passive effects can be used more effectively. Additionally renewable energy sources, like thermal solar heating and natural cooling by underground heat exchangers can be integrated into the energy concept in a a more efficient way.

Research projects and pilot studies in this field are are currently being supported by the 5th Energy Program of the Federal Government of Germany on “Innovation and New Energy Technologies” which was released on June 1st, 2005 (BMWi 2005). In this framework program the German research strategies in the energy field are outlined for the next years. The development of new systems, which improve the efficient use of the high quality energy, namely the use of exergy, is one key issue in building related research activities. The alliance project “LowEx” has been initiated in order to coordinate the different research activities, facilitate information exchange and develop a common approach for performance assessment and evaluation.

Background: The exergy concept and LowEx approach

Energy is at present an issue of political discussion, not only, but predominantly caused by the increasing costs of satisfying our energy demands. In this discussion, major emphasis is laid on the importance of energy efficiency and the need to “save” energy, implying the need to reduce energy “consumption”. This is not only part of everyday conversation but also in scientific discussions associated with energy and environmental issues. This colloquial terminology conflicts with the first law of thermodynamics, which states that the total amount of energy is conserved, even if forms of energy may change from one to the other. For this reason a scientifically correct approach which takes the second law of thermodynamics into consideration is needed to fully understand what is actually consumed. The exergy analysis provides this approach.

Improving the exergy flows in buildings, similar to other thermodynamic systems such as power stations, can help to identify the potential for increased energy efficiency. Comparative analysis show that calculations based solely on energy conservation and primary energy demand are insufficient for gaining a full understanding of all important aspects of energy transformation processes. Only an analysis based on exergy can quantify for example further improvements in the efficiency of boilers, whose energy efficiency is already close to 100%.

Exergy is the part of any energy flow which can be converted into some kind of high-quality energy such as mechanical work or electricity. Anergy, the remaining fraction, on the other hand, refers to the part of the energy flow which does not have this potential (e.g. low-temperature waste heat from power plants). Exergy can be regarded as the valuable part of energy, while anergy represents the low-value fraction (Shukuya and Hammache 2004, Schmidt 2004, Moran 1989). The “Low Exergy (LowEx)” approach demands matching the quality levels of exergy supply and demand, in order to optimise the use of high-value energy resources and minimise the irreversible dissipation of low-value energy into the environment.

The German alliance project

The German alliance project “LowEx” has been initiated to enhance the development of new and more efficient heating and cooling systems for buildings, which are also able to use more renewable and natural energy sources.

Objectives

The general goals of the project are specified in detailed objectives and fields of activity to be conducted during the course of this project.

Development of systems to use low exergy sinks and sources (e.g. ambient air, ground, and water).


Development of efficient heat carriers and transport systems, which consume low amounts of exergy.


Development of LowEx transfer systems in rooms that are able to operate with small temperature differences.


Development of thermally active façade constructions.


Development of LowEx storage systems.


System integration and operation of LowEx systems.

Structure

To cover all of these objectives, the alliance project “LowEx” has been subdivided into three separate fields of work. The first field covers building systems, the use of phase change materials (PCM) in the building construction is being investigated and new product developments are initiated. In the second field on supply systems, the application of so-called capillary tubes, also in combination with other technologies, is the primary scope of work. As the two main fields are not specific enough for a number of innovative solutions in sub-projects and pilots, a third field for integral projects has been defined, which covers a more general view on the holistic system approach and the application of the new technologies in building pilots.

Figure 1: The structure of the ongoing German alliance project “LowEx”

Currently, 24 sub-projects, with partners from leading German research institutes and industry, are being carried out within the framework of the alliance project “LowEx”.

Working area: Building systems

Based on the LowEx principles, new innovative HVAC system solutions are developed like storage systems, utilising so-called PCM (phase change material) in building construction and inside the HVAC equipment. With phase change materials, the latent heat storage effect, i.e. heat uptake, storage and release by melting and freezing of special materials at suitable temperatures (e.g. 24-26°C), is utilised. The PCM material can be encapsulated into microcapsules and integrated in building materials such as plaster or gypsum boards, or it can be packed in a macro scale into packages. By implementing PCM material in the building construction, the thermal storage capacity of the entire construction can be increased significantly. Thereby a dampening effect and increased comfort for summer conditions in light-weight constructions can be achieved. Another application is the integration of PCM storages in decentralized air handling units to increase the efficiency of night cooling. Some night coolness is stored in the system and released during hot summer days. These systems can be integrated in the construction of the façade.

An innovative approach is the development of new heat carrier fluids with PCM materials, so-called PCM slurries, to increase the storage capacity of the heat carrier. Energy for the fluid transport, e.g. electricity to operate pumps, can thereby be saved. The possible utilisation of PCM microcapsules in water or glycol, and PCM in emulsions, is being investigated. This technology also offers a new way of storing cold with PCM material, with suitable melting points.

Working area: Supply systems

The base technology for this working area is the implementation of so-called capillary tube systems. Thin plastic tubes of about 4 mm in diameter are placed in parallel to each other at a distance of about 15 mm and mounted together to grid systems, of about 2.5 m long and 1 m wide, with supply and collector tubes at the end. Thereby, a prefabricated and comparably cheap system can be achieved. These tubes can be used in thermally activated surface systems, such as floor, wall and ceiling heating and cooling systems, with a typical power output of about 80 W/m² for heating and 40 W/m² for cooling. Since the system is quite thin, it is possible to use it in retrofit measures, too. In the alliance project, applications such as active surfaces and active façade constructions, and air to water and ground heat exchangers are being investigated. The thermal activation of basements, by using the floors of cool basements for the cooling of offices, is also an application.

These capillary tube systems can also be used for horizontal ground heat exchangers to utilise the ground as a heat source or sink. Furthermore, the application, together with a PCM suspension as a heat carrier, is a current research topic.

Figure 2: Displacement of heat between rooms with different thermal loads (e.g. North-South, server rooms) with capillary tube systems.

Figure 3: Active exterior façade construction for active night cooling in combination with PCM storages in the building construction.

Integral projects

Main working items in integral sub-projects are the development of software tools for an energy/exergy assessment or for special studies, and dynamic analyses of complex systems. For these studies, the development of new models for innovative system solutions and components is needed and ongoing.

Figure 4: Results from a simplified tool for exergy analyses

Figure 5: Detailed dynamic model for system optimisation

Other integral projects working with demonstration activities are being set up as dissemination platforms, like the website of the project. Further information in German and details about the sub-projects and partners can be found at: www.lowex.info.

Other related activities

Research and development of LowEx systems is not only ongoing in Germany. Thus, an international working activity in the general framework of the International Energy Agency (IEA), the ECBCS Annex 49 “Low Exergy Systems for High Performance Buildings and Communities” has been initiated with participants from a number of countries. Furthermore, the International Society of Low Exergy Systems in Buildings (LowExNet) is an active and informal network of researchers in this field of interest. LowExNet members are working with exergy issues and have been presenting their results and findings in a number of workshops and seminars, mainly in the framework of international conferences within the field of building technology, building physics and building services. The LowExNet group offers a platform for discussion and information dissemination on the proposed activities. To strengthen and expand scientific collaboration in the LowEx field, a number of national and European projects have been either started or are in proposal (LowExNet 2006).

Summary / conclusions

As stated above, there is an obvious and indisputable need for an increase in the efficiency of energy utilisation in buildings. This is especially valid with regard to the great saving potentials in the field of buildings. Most of the regarded energy uses, e.g. the heating of rooms to about 20°C, are, from their nature, low exergetic. It can be shown that the energy concept alone is not adequate to gain a full understanding of all energy use processes in buildings. From that viewpoint, the exergy concept can close this gap and help to increase the understanding of energy flows. Energy manifests itself not only via its quantity, but also via its quality, its exergy.

All improvements and innovations of energy systems in buildings should be measured with the most efficient and best possible use of the energy quality. We should care more about the efficient use of high quality energy and try to save not only energy, but also exergy.

Acknowledgement

The authors would like to express their gratitude to the German Federal Ministry for Economics and Technology for their financial support in this project.