What fluid do I need for my process?
If there is demand every day of the week it is likely that all the solar steam produced can be used to reduce the consumption of the conventional boiler. In this case the simplest integration is to use the solar boiler as a parallel boiler with direct steam generation.
Direct steam generation in a solar boiler
The solar boiler uses a steam drum that separates steam and liquid water. Liquid water is sent to the solar field from the bottom of the steam drum. When this water passes through the solar field, it gradually evaporates thanks to solar energy. In this way, steam is generated inside the receiver tube. At the exit of the solar field a water/steam mixture is obtained which is led to the steam drum. There the water (which falls by gravity to the lower part) and the steam (which remains at the top) are separated. From the top the saturated steam is discharged at the same pressure as the steam generated by the conventional boiler. To replenish the discharged steam, the steam drum is fed with liquefied water from the condensate tank.
When there are days of the week where there is no consumption (for example on weekends), it is necessary to store the solar energy produced those days, since otherwise it would be wasted. Alternatives to store that energy depend on the fluid needed for the process.
Steam is a fluid that is not easy to store (due to its high specific volume), so intermediate fluids are usually used. To generate steam with intermediate fluids, a solar boiler is used in parallel with indirect steam generation.
Indirect steam generation in a solar boiler
To generate steam indirectly, an intermediate heat transfer fluid (HTF) is used. This fluid circulates through the solar field in a closed circuit (without mixing with steam). Depending on the vapor pressure required for the process, this fluid can be pressurized water or thermal oil. To generate the steam, a kettle reboiler steam generator or shell-tube exchangers is used. The advantage of using an intermediate fluid is that it can be stored very economically using insulated tanks. Storing allows not to waste solar energy when the factory has no consumption.
When a significant part of the steam generated by the boilers is used to heat up water, it is possible to take advantage of the solar energy when there is no consumption. To do this, the solar boiler must perform hybrid heating. Hybrid heating allows the solar boiler to be used to generate steam and not waste solar energy on days when there is no consumption, storing it in the form of hot water.
Hybrid heating steam / hot water
Suppose, for example, that our factory needs steam only from Monday to Friday, and that this steam is largely used to heat water. Under normal conditions, the steam that the plant generates during the weekends would be wasted, since storing the steam is complicated. To avoid this, the steam generated during weekends is used to heat water that is subsequently stored in insulated tanks. This hot water can be used during the week without problem. For hybrid heating to make sense, there needs to be a centralized point from where hot water is distributed (a reservoir or set of tanks). Otherwise it would be necessary to duplicate the entire distribution system.
If the process uses hot water or thermal oil, the solar boiler can be used in parallel, or to preheat the process fluid. In both cases, depending on the demand profile, storage in thermally insulated tanks can be used. To see the difference suppose for example that in the factory I use a gas boiler to heat oil from 140 ºC to 220 ºC.
Heating in parallel
When the solar boiler is used in parallel, its operation is similar to that of the conventional boiler. The solar boiler is fed with the cold fluid (oil at 140 ºC) and this is responsible for heating it to the operating temperature (240 ºC). The hot fluid is sent to the collector and from there to the processes. Heating in parallel is used when the operating temperature may vary.
Preheating of the process fluid
When the solar boiler is used for preheating, it is usually done through a heat exchanger. In this case the cold oil comes at 140 ºC and when it passes through the exchanger it comes out at 210 ºC. With this we get the conventional boiler to work less since the temperature difference is less. Preheating is often used in factories with high consumption or when the process temperature cannot deviate from a fixed value.
In addition to classical fluids (steam, water and oil), industrial processes use a wide variety of fluids. Solar boilers can perform the heating of this type of fluids in two ways: direct heating and indirect heating.
Direct heating
In direct heating the fluid is circulated through the receiver tube of the solar field. The solar radiation reflected by the mirrors is used to directly heat the fluid. Direct heating is possible as long as the characteristics of the fluid allow it. There are, for example, different research projects using direct heating with gaseous fluids (e.g. air, CO2, etc.), and organic fluids (e.g. sludge, wastewater).
Indirect heating
In indirect heating, the solar field heats the process fluid using an intermediate heat transfer fluid (HTF). The HTF circulates through a closed circuit through the tubes of the solar boiler, and is then used to heat the process fluid by heat exchangers. This system is quite common for air preheating in dryers for example.