Research areas of focus at Sustainable Energy Research Laboratory are:
Problem #1: Carbon deposition and reactor clogging
Problem #2: Intrinsic losses in energy conversion due to transient nature of solar energy
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Upon the decomposition of natural gas into hydrogen and carbon inside the solar reactor, carbon particles tend to move towards reactor window and deposit on the quartz window. The rest of the carbon particles either deposit on reactor walls or move towards the exit all together. Carbon deposition on the quartz window blocks the incoming solar radiation and therefore drops the temperature inside the reactor. As for the deposition at the exit, those carbon particles agglomerate and completely block the exit causing the reactor to explode.
Solution to reduce carbon deposition
We have approached to this problem by investigating the flow behavior inside a solar reactor for this two-phase thermochemical reaction. In order to understand the flow behavior and track the carbon particles inside the reactor, we have applied our successful CFD model to our aero-shielded reactor concept.
In this reactor concept, natural gas main flow is injected through impeller disk jets from the top center of the reactor with a 45° angle at a flow rate of 7m/s creating a strong vortex concentrated in the middle of the reactor. With this reactor configuration, we obtained a laminar flow shield covering the interior walls of the solar reactor as shown in Figure 1(a) and strong vortex-or cyclone-in the core as shown in Figure 1(b).
Problem #2: Intrinsic losses in energy conservation due to transient nature of solar energy
Re-radiation losses and inherently transient nature of the solar energy are the main reasons for intrinsic losses in energy conversion in any solar reactor. When solar energy enters into reactor as a source of high temperature process heat; incident radiation needs to be concentrated over a small surface area, the inlet of which is called the aperture. The image of incoming solar radiation over the aperture can be approximated by a Gaussian distribution where the solar radiation inside the reactor varies by the peak value and aperture size. However, because of the transient nature of solar energy, there is a critical need for proper control to maximize system efficiency under field conditions. Up to date, all solar reactors have been made on the assumption of homogenous and instantaneous temperatures inside solar reactor, e.g. fixed aperture. It should be noted that intermittence in the incident flux entering the reactor is a product of the environment, geometrical design of the concentration system, tracking algorithms and overall concentration ratios. Furthermore, wear and fouling of concentration surfaces and of the aperture window also affect maximum achievable flux densities. These factors combined are present within any solar concentrator and each serve to decrease efficiency. Therefore, it is important to design a system that allows the reactor to respond to environmental factors in order to maintain semi-constant temperatures inside the reactor.
Solution to control intrinsic losses
It is important to keep the reactor temperature constant so that the process efficiency remains constant regardless of weather conditions. In order to do that, a variable aperture sensible to changes in weather would help in maintaining semi-constant temperature inside the solar reactor.
By determining the output of the reactor with a dynamic aperture and changing flux conditions, we can see the impact of variable aperture size. We know that the absorption efficiency is the ratio of difference in how much power is intercepted by the aperture and how much power is reradiated through the aperture due to the cavity temperature to the total amount of power reflected towards the reactor by the concentration system as shown in Eq. (1):
where eff and eff are the effective absorptance and emittance of the receiver, Paperture is the amount of power intercepted by the aperture, Pin is the total power reflected by the concentration system, A is the aperture area, is the Stefan-Boltzmanns constant, and T is the internal temperature of the cavity. The energy used during the chemical decomposition of methane is defined as the product of the required change in enthalpy and the mass flow rate. Convective losses can be modeled based on a desired isothermal temperature and room temperature environmental operating conditions under natural convection. Internal wall temperatures can be considered as the same temperature of the cavity temperature. Heat is conducted through the cavity, through the insulation, and then dispersed through natural convection. Based on this model, which is described elsewhere in detail [10], variable aperture size indeed help in maintaining semi-constant temperature inside a solar reactor.
For example, if we want to capture lets say 4kW power inside the reactor regardless of changes in weather conditions, we can refer to Figure 3 to find out what should be the aperture size. If the incoming power is 18.9kW, we need to shrink the aperture size to 4cm approximately. Or if the incoming power is 11.3kW, then the aperture size should be approximately 6cm to capture 4kW. If the incoming power is too low because of clouds or dust storm etc., such as 5.67kW, then the aperture size should be enlarged to 10cm approximately to capture 4kW. This sequence shows us that by changing the aperture size according to the changes in the incoming solar flux, we can maintain semi-constant conditions inside the reactor. It should be noted that, the aperture, by definition, is at the entrance to the reactor and will experience a high degree of solar influx. Active cooling would be needed in order to maintain operational capabilities when the aperture is occluding a large part of the concentrated solar power.
It should also be noted that some of the incoming radiation will be lost when the aperture size is changed. However, what is more valuable than intercepting the maximum amount of radiation is maximizing the net amount of energy transferred into the reactor. The variable aperture allows the reradiation energy to be modulated at a cost of intercepted energy. The exact aperture size is dependent on the distribution and intensity of the concentrated solar radiation, desired internal temperature, and actual internal temperature. Because of these temperatures, reradiation losses, depending on aperture geometry, will take precedence over conductive and convective losses, where the radiation losses are proportional to the aperture area and internal temperature to the fourth power.
Therefore, efficiency is enhanced because controlling the aperture enables the control of reradiation losses and intercepted radiation. At some point, again depending on the total concentrated flux intensity, distribution, and aperture size, there is a maximum amount of net power available. This maximum changes as the flux intensity changes, e.g. because of weather, fouling, etc. By changing the aperture size, we can maintain, or more closely follow, the maximum efficiency throughout various operating conditions.
If we take a look at the impacts of this mechanism on solar cracking of natural gas to produce hydrogen: essentially the hydrogen production is bounded by the net power received by the system minus the convective and conductive losses. The general end goal of solar thermal cracking research is to provide an alternative approach to traditional hydrogen gas production methods. Therefore, variable aperture mechanism may offer a good alternative to maintain semi-constant hydrogen production with zero emissions regardless of weather conditions, except for during the night of course.
Once completed our CFD and Optics simulations, we created three dimensional animation of the variable aperture mechanism for the solar reactor as shown in Figure 4.
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