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               Heat Flux Principle & Method to Accurately Measure U-Value
               Heat Flux Sensors are based on the Seebeck effect. When heat passes through the
               sensor, the sensor generates a voltage signal. This voltage signal is proportional to
               the heat passing through the sensor. greenTEG Heat Flux Sensors can resolve heat
               fluxes < 0.01 W/m2.

                In the field of building physics, U-Value is the term used to describe the
                heat transfer coefficient of a building element (e.g. wall or window).
                U-Value describes the insulation quality of a building. The unit of the
                U-Value is W/(m²K). The smaller the U-Value, the better the insulation
                quality of the building element.

                How to measure U-value? The greenTEG measurement-approach uses a
                heat flux sensor and two temperature sensors. This approach is standardized in
                ISO 9869, ASTM C1046 and ASTM C1155. This is the only method which delivers
                reliable quantitative in-situ information about a building envelope. Placing sensors inside and
                outside of a building, transmitting the data to nodes connected to the internet via a secure
                gateway and then performing analytic calculations to convert to energy billing costs will provide a
                convenient way for building owners to cost-justify investments that will reduce energy waste.
                Why Is This an Advantage?
                Energy audits that provide accurate information on ways to reduce waste are required to validate
                subsidies, rebates and financing of energy saving improvements. Currently the time and expense
                 required, as well as personnel, certifications and equipment needed to conduct reliable audits limits

                 the number of audits that can be completed. Furthermore, the methods now being used do not
                provide reliable real-time data. For example:

                1. Thermography (i.e. infrared imaging) shows thermal radiation of an object as an image showing spots with
                higher and lower radiation. Thermography helps to understand overall quality of a building envelope and
                 identify thermal bridges and sections with inadequate insulation. However, it does not produce quantitative
                data (e.g. U-value in W/m2K) that can be used to interpret energy loss. Therefore, this method can only be used
                 to roughly approximate the U-value.

                 2. Multiple temperature measurements - By synchronizing three or more temperature sensors inside and outside
                 of a building element, it is possible to calculate the heat flux indirectly, and from this information, derive the U-
                value of a building element. While this method generates quantitative data, it is not usable for in-situ
                 measurements. To apply this method, a minimum temperature difference of 10 °C between the inside and
                outside temperature is required. Such temperature differences do not occur very often in most regions, and are
                 most likely not achieved continuously throughout the year. Moreover, both the inside and outside conditions
                must be constant during the measurement period and no solar radiation is allowed. These requirements make it
                 very hard to obtain reliable data.

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