InnoGOK – Investigation of the energetic and ecological usability of solar radiation on urban spaces and paths

Starting point/Motivation

Concerning the consequences of global climate change and consequently increasing urban heat islands, the question arises how to provide suitable renewable energy out of public spaces, while maintaining a comfortable urban microclimate. This is particularly important, because health and general well-being of inhabitants, working and living in buildings as well as in urban outdoor areas, depend decisively from urban microclimate conditions.

The operative temperature of cities has to be lowered sufficiently by reducing urban heat island effects to retain comfort and physical fitness criteria of city inhabitants.

On the other hand, environmentally-friendly measures have to be found to fulfil those comfort criteria in combination with lowest possible energy demand.

Contents and Objectives

The aim of this research project is to work out the feasibility of the use of solar energy, collected on the huge amount of existing horizontal ground surfaces in cities (highways, squares, parks, sports grounds). This option is seen as an alternative for conventional thermal solar energy systems. At the same time the potential for reducing urban heat islands was shown. The energy gained from urban surfaces is thought to be used in the close surroundings (e.g. for heating water, or keeping traffic routes clear of ice), in combination with intelligent energy storage methods for time-delayed use of the gained energy while at the same time, this discharge leads to a reduction of surface temperatures.

Methods

The HTL (higher technical college) in Krems is equipped with outdoor test surfaces and energy storage devices (ice storage unit, ground heat exchanger). These testing facilities have been modified and adapted with measuring devices for a comprehensive monitoring. Based on the evaluated results, mathematical connections have been worked out for further calculations and parameter studies.

Results

The results suggest a high potential for heat exchange functionalities especially for the existing asphalt surface. Heat outputs up to 300 W/m² are possible on very sunny days whereas the existing concrete test surface is able to provide up to 200 W/m². At the same time it is possible to reduce the surface temperatures up to 10 K (asphalt) resp. 5 K (concrete) as a result of the heat discharge which indicates the potential for reducing urban heat islands.

The asphalt/concrete heat exchanger element can also be used as a free cooling component if the reduction of urban heat islands is not the main focus.

In summer the amount of discharged heat up to 4 kWh/m²d or 250 kWh/m² is technically possible to store the heat seasonally in large earth storage or borehole tanks. In this case a correspondingly large amount of cubic meters of earth from about 1.000 m³ is required in order to reduce the heat losses against the storage capacity. The phase-shifting use provides a broad field of application.

Prospects / Suggestions for future

In this project, significant potentials for the reduction of urban heat islands by heat discharge from horizontally sealed areas were figured out. The numerous possible applications are given from a technical point of view. In a next step the system has to be investigated in an economically and ecologically point of view.

Transient simulations have to be carried out in regard to the wide fluctuations of the microclimate as well as the thermal connection of the absorber to the soil. This enables quantitative statements for the feasibility in a real environment. Particular attention must be paid to a high flexibility in respect to subsequent installations or restorations. Bandwidths for the simulation parameters have to be considered as well.