A flat plate solar energy collector is a passive device that harnesses solar radiation to heat a working fluid. The working fluid can be water that is directly used or another liquid that then transfers the absorbed energy to water. This later system would be used when there is a chance for the water to freeze if it were exposed to external temperatures.
As the reader will find, these systems are very low technology, however, they are very efficient. Efficiencies, in terms of how much insolation is transferred to water, are over 50% which is much greater than photovoltaic devices and are even more efficient than coal-fired power plants which are around 35% efficient. Thus, although these devices are not technologically appealing they are well worth installing at your residence or place of work due to their efficiency.
Perhaps they should be the first solar powered device one should purchase since their price is fairly low and they will have a long service life. In addition, they require low cost maintenance, unlike solar panels. Maintenance can be high for photovoltaic devices because the power inverter, which converts DC to AC current, must be replaced at least once during the life of a photovoltaic system, adding a cost of between $2,000 and $20,000 (depending on the system size) over the device’s entire usable life.
So, solar thermal hot water heating systems that directly convert energy from the Sun to a useable form seem more cost effective. They should save the average household on the order of $200-$300 per year and take about 10-15 years to pay off the system, which will last for approximately 20 years.
Although the above argument points to having a solar thermal hot water system, there are some that argue it is cheaper and more effective to heat water with photovoltaically generated electricity and an air source heat pump (see the reference for Holladay at the end of this chapter). This argument does not follow the general rule-of-thumb that one should never use high quality energy to make low quality, if at all possible. In other words, use electricity to power electrical devices rather than to heat water. Of course, the decision as to whether solar thermal or solar photovoltaic is the right choice depends on your location, water demand, system cost, etc. and careful decision-making should be performed. By the end of this chapter the reader should be able to make that informed decision.
Solar Energy, An Introduction, First Edition, Michael E. Mackay © Michael E. Mackay 2015. Published in 2015 by Oxford University Press.
A significant deviation from other textbooks and analysis techniques is made in this chapter. The heat removal factor (FR) and other parameters are often introduced and used to estimate the useful heat (Qu) transferred to the working fluid as it flows through the pipes. The useful heat is defined below and will be estimated via basic engineering heat transfer calculations. The heat removal factor technique finds Qu with charting techniques and/or correlations that have been developed over a period of decades. Here the reader is introduced to the basic principles that can be used to design a solar hot water heating system from the beginning.
This decision was made for two reasons. Firstly, it is believed that the correlative approach was introduced because radiative heat transfer has a difference of T4 between surfaces, which complicates hand calculation of design equations and which almost always involves iteration. The reader now has powerful calculation tools that can easily solve coupled algebraic equations involving non-linear terms. Thus, the base equations are given rather than charts and correlations. Secondly, new materials are being developed and this could change how these systems are designed. For example, an effort exists to make flat plate solar collectors out of plastic which would most likely prohibit the use of older correlations. Thus, the reader would be faced with a challenge that might seem insurmountable in designing such a system. It is hoped that by the end of this chapter the reader will have a good grasp of how to design a contemporary system, and that knowledge can be generalized to more innovative devices. Dimensionless numbers and correlations between dimensionless numbers will be used to design the devices in this chapter; if the reader is not familiar with these concepts, reading Appendix C may help one to understand what they are and why they are used.