With the natural growth of the solar industry in South Africa, we are now able to access materials and equipment that were previously unavailable. As consumers are becoming more and more aware of solar water heating, the demand for larger, more advanced systems has grown. Integration of various heat sources and heat demands is now possible, using carefully designed and sometimes patented hydraulic components.
Changes to the National Building Regulations have also created demand for integrated solar thermal systems that incorporate additional heating demands such as swimming pools and space heating. The integration of multiple heat sources and heating demands has in the past been a rather complex matter, since the design and control of these systems requires some careful planning. Access to more efficient pumps has enabled these complex systems to be finely managed to ensure the most effective delivery of renewable energy to the right place, at the right time, at the right temperature.
Fortunately for us (or unfortunately, depending on your viewpoint), we are able to learn from others in the global industry and we now have access to solutions that make the task of implementing world class solar thermal solutions much easier that is had been before.
A number of fundamental principles are paramount in the design and effective execution of these systems:
• Fresh water heating technology • Stratification
• Flow modulation
• Monitoring
Figure 1 Plant room incorporating efficient technologies
Let us look at these principles in more detail
Fresh Water Technology
The term is slightly misleading and derives its origin from the German“Frischwasser”, which is used to describe a component that provides instantaneous hot water, on demand, without storing the heated water. In these fresh hot water systems, the heating of the sanitary hot water takes place in a heat exchanger that draws its heat from the solar (or otherwise) heated buffer tanks. Hot water to be delivered to the demand points is never stored or kept in a heated state in a sealed vessel. The buffer tanks therefore become nothing more that static thermal batteries that store kilowatt hours in an inert medium, water. The water in these buffer tanks remains heated and insulated in a closed sealed loop to the various heat exchangers, allowing the system to draw from this stored heat as and when required.
This has a number of advantages:
Reduced heat loss from distributed small volume storage
More efficient heat delivery
Diverse temperature delivery from one integrated system
Significant reduction in turbulence in the stored water (buffer)
Dramatically reduced pathogen growth and reduced health risk
Figure 2 Legionella pneumophilla (National Health Service UK, 2012)
The last point above is perhaps the most crucial of all, since we live in a society where the average consumer may very well be living with a compromised immune system and would be much more vulnerable to contracting respiratory disease. The main culprit in hot water is a bacterium of the genus Legionella, which is spread in fine water droplets (aerosols) and can lead to an acute respiratory condition known as Legionnaires’ disease (World Health Organisation, 2007). This bacteria thrives in stagnant water in temperatures of between 30°C and 50°C, and can survive in temperatures beyond these limits. The use of fresh water heating systems provides a means to deliver hot water without the risk of bacterial incubation, thereby ensuring safe, clean, uncontaminated hot water. This is especially important in health care and hospitality applications where infectious diseases can spell disaster.
For detailed information about Legionella in the South African context, consult the recently published South African National Standard for Legionella Control, published 13th May 2013 (SANS893) (Ecosafe, 2013)
Stratification
The concept of stratification in hot water systems refers to the fact that layers of water at various temperatures naturally separate due to density differences, with the hottest water at the top and the cooler water at the bottom. The use of plate heat exchangers to transfer the heat to the required heating demands allows low circulation rates, thereby reducing turbulence and encouraging the stratification effect. This technique is often referred to as the ‘low flow’ or ‘single pass’ and is characterised by mass flow rates of approximately 5-20kg/m2h (AEE – Institute for Sustainable Technologies, 2009). There are a number of distinct advantages to stratification:
• Target temperatures at the top of the buffer are rapidly achieved
• Solar collector efficiency is increased due to lower inlet temperatures
• Reduced auxiliary heating demand
• Lower mass flow rates mean smaller pipe dimensions and also smaller pumps can be used The overall effect of correctly applied stratification is a reduction in the total energy required to
run the system, and a resultant reduction in total system cost (German Solar Energy Society, 2010)
Flow Modulation
Flow in piping is a much misunderstood and highly dynamic issue within solar heating design in South Africa. The vast majority of designers/installers do not consider the flow rate when designing or implementing larger scale solar thermal systems. In fact, many of them interviewed indicated that they gave it no consideration at all, beyond ‘is the fluid moving or not’ (Students, 2013).
The truth is that flow rates in solar thermal systems are absolutely crucial and can make the difference between warm water and hot water, no matter how cleverly contrived the rest of the design may be.
For example:
• High flow rates use high power pumps, increase electrical consumption and friction losses. • High flow rates may cause the disruption of stratification within the buffer tanks
• High flow rates accelerate deterioration within heat exchangers
Whereas:
Low flow rates reduce the electrical energy required to run pumps and also reduces the
friction losses in the piping.
Low flow rates allow effective stratification within buffer tanks
Low flow rates increase the efficiency of solar thermal collectors, by lowering the collector
inlet temperatures.
Low flow rates increase the temperature at the outlet of both heat exchangers and solar
collectors, thereby allowing the target temperature required in the buffer tanks to be reached more rapidly. (German Solar Energy Society, 2010)
Thanks to the demand for more efficient pumps in the EU and elsewhere, we are able to access pumps and controls that allow speed management of single phase hot water circulators. By implementing a control strategy that dynamically adjusts the flow rate according to temperatures and temperature differences, the total system efficiency is increased, not only as a result of reduced pumping power, but as a result of more efficient solar harvesting (German Solar Energy Society, 2010). In its simplest form, a solar differential controller measures the temperature at the hottest point in the system, compares it to the coldest point in the system and adjusts the pump speed up or down accordingly. In other words, the pump will dynamically increase or decrease its speed as the solar input varies throughout the day. This is particularly important in a climate where summer thunderstorms are prevalent, since the solar circuit will adjust for the reduced radiation during the storms, thereby ensuring efficient solar harvesting.
Flow modulation is also critical in fresh water heating systems, which could easily be called the inverse of solar heating circuits – instead of changing the buffer with thermal energy, they discharge thermal energy in a controlled manner. Flow modulation is again very important, since the variable speeds of the pumps on either side of the fresh water heating system heat exchanger will ensure that the target temperature is immediately reached.
In addition, variable speed pumps form an integral part of energy efficient hot water circulation in buildings. By reducing the rate at which the water is moved in the circuit, the heat losses are reduced and the pumping power is consequently reduced.
Many advanced controllers, notably the range produced by Resol, Germany, are able to handle multiple circuits at once, and can be expanded to accommodate multiple stores (buffers) and multiple collector arrays, all with fine pump control included.
Monitoring
In order to deliver the best results from an efficiently designed and correctly installed solar thermal system, one cannot expect to operate on the ‘fire and forget’ principle. On the contrary, by monitoring the performance of the system post-commissioning, the set points, flow rates, pressures and other important parameters can be adjusted to reflect the operational realities of the system within its installed context.
Fortunately, many of the control systems allow data logging and display, using very basic components and a reliable data (internet) connection. In some cases, such as with Resol and their vBus. net service, the framework upon which one can build a monitoring portfolio is provided free of charge.
Monitoring of almost all states and values in the solar thermal system is possible, with the following being the primary metrics:
• Temperature
• Temperature difference
• Heat quantity delivered (kWh) • Flow rate
• Volume delivered
• Pressure
• Radiation
• Run time
• Clock time
This may seem to many to be an‘over engineered’solution, but the reality is that energy delivery is greatly increased if the setpoints are adjusted once the system is commissioned (German Solar Energy Society, 2010)
An important fact about monitoring solar thermal systems is also often overlooked here in South Africa. By allowing insight into the performance of a particular solar thermal system, the installer is showing confidence that the system will deliver energy as predicted, according to the client’s expectations. Publicly displayed data can be a double-edged sword, and one has to be very sure that one’s design is correct before exposing oneself to criticism. In other words, those that are brave enough to share the recorded data with their clients, are confident enough that the system will do as it was designed to do – deliver heat consistently and efficiently.
Recent experience (Author, personal experience, Midrand, 2013) showed that by adjusting various temperature set points within the solar differential controller of a large scale solar thermal system, delivery of energy improved by an estimated 10%. Additionally, the live display of data allowed the author to monitor the results of the adjustments to ensure that in fact the changes were positive.
The graph below clearly shows the increased temperature after certain adjustments were made to the system:
Figure 3 vBus.net display of recorded data (SEG Solar Energy (Pty)Ltd, 2013)
The adjustments referred to above are only one of the many improvements that have been, and will continue to be made to improve the performance of the system. The most positive spin-off of this initial monitoring is that the installer of this system has been awarded a second contract on site, with an expected volume of 12 000l.
For the client, the benefit has been not only the improvement of performance of the system, but it has highlighted the excessive water consumption on site. The result is that water usage is expected to drop dramatically in 2014, once the installation of efficient shower heads is completed.
Further developments in these systems will allow contractors to manipulate the set points of solar thermal systems remotely, with the use of a pc or tablet device. In other words, immediate adjustments can be made without a physical presence on site, and the work performed can be billed accordingly, without the service provider ever leaving the office.
Products and Solutions for Advanced Solar Thermal Systems
Now that we have established that fine control and management of solar thermal systems delivers better results and higher energy gain, how can we implement these?
In all cases, we would seek a solution that meets the following basic criteria:
• Efficient
• Affordable
• Robust
• Simple to install
Fortunately we now have access to a number of solutions that combine all the essential elements that we have mentioned so far, both on the input and delivery side of the system.
Stratified Charging Module SLM120XL
Marketed by SEG Solar Energy (Pty)Ltd, this streamlined solution is suitable for both new and retrofit installations. The SLM120XL unit is able to handle up to 120m2 of solar thermal collector area and incorporates a Resol differential solar controller and energy efficient variable speed pumps. (SEG Solar Energy, 2012)
Figure 4 Stratified charging module SLM120XL (SEG Solar Energy, 2012)
In addition to the main function of the unit, the following information can be recorded and displayed:
• Flow rates and volumes
• Temperatures at all sensor positions • Heat quantity produced
• Error states
Fresh Water Module FWM225XL
When it comes to delivering uncontaminated, instantaneously heated water, the Fresh Water Module FWM225XL is the uncontested leader in the game. This patented product was the result of research and development performed by individuals who subsequently built and now operate the world’s largest manufacturer of flat plate solar collectors.
The FWM225XL module is designed to provide instantly heated hot water, while drawing the heated fluid from the buffers and simultaneously providing hot water recirculation in the building. As previously described, this component uses a plate heat exchanger and variable speed pumps to ensure the most efficient use of stored heat and consistent delivery of clean hot water, without the risk of bacterial contamination. (SEG Solar Energy, 2012)
With the help of the controller fitted to the unit, various data points can be read and displayed, such as:
• Temperature • Flow rate
• Run time
• Pressure
Figure 5 Fresh Water Module FWS225XL
By combining the aforementioned devices with the appropriate storage vessels, the result
would look something similar to the representation in the diagram below:
Figure 6 Large scale SWH system with stratified charging and fresh water technology (SEG Solar Energy, 2012)
Conclusion
Solar thermal systems are fast achieving their correct place in the South African HVAC industry – that of primary energy source – and no longer the ‘alternative’
The question is no longer “does solar work?” but rather “how can you maximise the delivery of solar energy” in your project today, and into the future.
Given the range of efficient solutions available, it is inconceivable that anyone would be hesitant to invest in the cleanest source of energy our species has ever seen. Solar simply works.
Source: Sustainable Energy Resource Handbook Volume 5
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