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FS41 - The control algorithms for optimal operation for a solar-wall system for domestic buildings

Written by: CIC Start Online

"The control algorithms for optimal operation for a solar-wall system for domestic buildings " by Heriot Watt University and Invisible Heating Systems Ltd (IHS)


With the world's most ambitious greenhouse gas reduction targets to cut the nation's CO2 emissions by 42% by 2020, Scotland is working hard to meet this target, by increasing its generation of renewable energy, improving energy efficiency and applying fiscal drivers from increasing fuel prices to implementing feed-in-tarrifs for renewable energy generation.

In building sector a big challenge is fuel poverty. In 2002 there were an estimated 286,000 households (13%) who were fuel poor. Of these, 24% (69,000) were in extreme fuel poverty (i.e. would have to spend more than 20% of their income on fuel to maintain the standard heating regime). This year this figure has risen to over 35%. Moreover many of them are living in properties that are “hard-to-treat” buildings for those common energy efficient measures. To face this problem, organisations such as Changeworks, Historic Scotland and Edinburgh Council have elaborated a guidance to apply renewable technologies in traditional and historic homes, including the most common system, solar water systems.

The efficiency of these solar water systems decreases greatly when the heat store, normally a cylinder is full and heat is not extracted during daytime when sun is strong. Now the common solution is to increase the size of the cylinder or add another cylinder, using a larger heat store to deal with the swift between the free solar heating supply and heating demands. However many of these tenements and the old properties built before 1910’s have limited space for extra heat storage but they do have solid internal walls and therefore heavy thermal mass, which has recently been rediscovered for their potential in stabilising and moderating the indoor thermal climate. The solar-wall system, in part developed using a previous CIC Start Online feasibiltiy grant, is designed to use the solid internal walls, instead of an extra cylinder, as the secondary heat store. The study confirmed that a solid wall can save over 70% of the heat input during the daytime and release gently the heat into the rooms on its two sides during the night-time. The heat input in that study was based an ideal solar panel collecting solar heat at full capacity. In reality the heat collection efficiency depends many factors, including how the wall store is connected with the conventional solar-cylinder system.

There is a lack of research to date that provides guidance for these layouts and their control algorithms. It is obvious that the efficiency of such systems integrating multiple heat exchange units inevitably depends on an individually designed control algorithm. Designing such a control program can be done in various ways. Computer modelling has proved as the cheapest method to simulate a large number of combinations of working conditions, including weather, occupancy patterns and system layouts.

This study will develop a computer model using TRNSYS to simulate the operation under a number of possible combinations of weather conditions, occupancy patterns, and operation modes. The model will be validated by measured data acquired from a mock-up of a real sized similar system in the School of the Built Environment of Heriot Watt University under the late October weather conditions. This modelling exercise will carried out to develop an optimised control algorithm, including a logic flowchart and control variables.

The outcome of this study will be a set of control algorithms with recommended values for control variables for the selected conditions that are common in Scottish tenements. It will also ptovide an estimate of the cost of the system and the energy saving.

Key words: refurbishment, domestic buildings, renewable energy, hot water solar panels, environmental modelling


Dr Fan Wang, Heriot Watt University

For Invisible Heating System

The outputs of this study will be disseminated in due course.


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