Testing evacuated tube solar collectors: creating equipment for efficient, economical conversion of solar radiation.

Thermomax Ltd. is a manufacturer of equipment for converting solar radiation into thermal energy. Our evacuated tube solar collectors are used to produce clean energy for domestic and industrial hot water, space heating, and cooling and seawater desalination. At our manufacturing facility in Bangor, Northern Ireland, we have installed two new test facilities--one for outside tests under real conditions, and an indoor solar simulator. The new facilities have significantly reduced the time and cost of product development and have been useful tools for ongoing production quality assurance. Using the solar simulator, we can perform tests all year round, no matter what the weather conditions are outside.

Building a system

Each of the test systems needed to be sufficiently reliable for month long operation, the typical length of time we test our collector designs. We opted for programmable automation controllers, rather than PLCs, because the digital system offered greater flexibility for handling I/O plus motion, vision and communication streams.

Each facility was constructed with PC-base& fully automated test systems using FieldPoint-based hardware and software, including National Instruments FP-1601 network modules, relay, counter/timer, analog input, output, and thermocouple modules, and NI LabVIEW DSC and LabVIEW software. We also included a two-axis motion system controlled by an NI PCI-7342 motion controller and an NI UMI-7772 interface using LabVIEW. We employed six FP-1601 network modules on the outside test rig controlling and acquiring data from 212 channels. Due to the high channel count, we employed the LabVIEW DSC Module to reduce the development time for the control and data acquisition system and store the data in an easily accessible database. We can test up to four collector systems at any one time and subject them to one of nine test sequences.

We employed an additional FP1601 network module on the solar simulator to acquire and control 34 channels. We used the two-axis motion system to acquire an irradiance map of the simulator lamp array prior to each test. We used a LabVIEW program to perform all the control, data acquisition, and analysis enabling automated efficiency tests on the collectors to BS EN 12975. We had overcome a unique range of challenges at each test facility. On the outside test rig, we had a large number of channels to deal with and data had to be recorded over months and even years. In addition, if the energy from the collectors was not used, they could become extremely hot. As a result, we incorporated alarms and safety interlocks into the system. We used the LabVIEW DSC Module, which made the design and implementation of the control system relatively straightforward. We used the tag configuration editor to interface with the FieldPoint channels via the Ethernet to considerably reduce the amount of code writing. Using the tag configuration editor, we easily implemented channel scaling, deadbanding, and alarms. The NI Measurement and Automation Explorer (MAX) software also proved to be extremely useful. During commissioning, we used MAX for low-level control of the FieldPoint hardware without having to write any code. When the system runs MAX, it provides a simple way to view and export data from the citadel database using the historical viewer.

On the solar simulator, we had a much smaller number of channels, but the test requirements of BS EN 12975 demanded precise control of the temperature and mass flow rate of the system fluid. To achieve this, we installed two Honeywell control valves, one to control the coolant flow to a heat exchanger for temperature control, and the second to control the mass flow through the collector under test. We used the advanced PID VIs in LabVIEW to generate the control signals to the valves via a FP-AO-V10 two-channel analog output module.

Using the advanced PID VIs, we implemented bumpless manual to automatic and automatic to manual control. Our second challenge was to develop a two-dimensional mapping system to precisely measure the incident irradiance on the collectors prior to each test. Initially, we attempted to acquire the map manually, but this required two operators and a significant amount of time. As a result, we decided to install a two-axis motion system to move the pyranometer and record the irradiance. We used an NI PCI-7342 motion controller and an NI UMI-7772 motion interface to control two stepper drives and motors. The system moved a Kipp & Zonen CM11 pyranometer, which is connected to an analog input channel on the FieldPoint bank, and records a 200 point map.

During a typical test run, the LabView software both controls the test module and compiles all the data taken from the sensor array on the collector and other test points. The same software can then perform the analysis, greatly simplifying the operation.

Results Using NI Products

By using NI hardware and LabVIEW software, a completely integrated solution could be constructed quickly and easily. The two test facilities have proved to be very successful and are in use daily. Because the development time for new products has been considerably reduced, we no longer need to send prototypes to European test institutes, which has reduced our development costs. The systems were instrumental in reducing the time to market for our new range of collector manifolds. We continue to expand the range of applications of the systems to quality control and marketing. In addition, our customers can view real-time testing of their products via the Web.

Circle 121--rhermomax Ltd., or connect directly at www.rsleads.com/502df-121

Circle 122--National Instruments, or connect directly at www.rsleads.com/502df-122