Smart DC Power Supply and Roll Polling Program

The first EKG (E-board) in China:

Zhuang, Y.F., Zou, W., Wang, Z., Tan, X., Hu, P., Hu, S., Yan, Y., Wang, Y., 2012. Electrically conductive PVD. Chinese Patent. Grant Number: ZL201210197981.4.

The first field trial using E-board in China:

Zhuang, Y.F., Huang, Y., Liu, F., Zou, W., Li, Z., 2014. Case study on hydraulic reclaimed sludge consolidation using electrokinetic geosynthetics. In: 10th International Conference on Geosynthetics. DGGT, Berlin, Germany.

After the field trial, it was found that power requirement, instead of energy consumption, is the biggest challenge for field application using electro-osmosis. And I believe this is the case for most applications using electrokinetic technique in Geo.

Zhuang, Y.F., 2015. Challenges of electro-osmotic consolidation in large scale application. In: Conference of Geosynthetics 2015. IFAI, Portland, Oregon, USA, pp. 447–449.

Therefore, I have been focusing on Smart DC Power Source and looking for a way to reduce both power requirement and energy consumption. Roll Polling Program seems to be a solution. It reduces the requirement of current intensity to 1/3; it also decreases the energy consumption to less than 1 kW⋅h/m3. Following contents are direct quote from the publication:

Zhuang, Y.F., 2021. Large scale soft ground consolidation using electrokinetic geosynthetics. Geotext. Geomembranes 49 (2021) 757–770.

Electro-osmosis requires direct current and for safety reason only low voltage can be applied into field. Field trials showed that DC voltage input into ground should not exceed 80 V for safety reason (Zhuang et al., 2014; Zhuang et al., 2018). Therefore, DC power source is needed to convert alternating current with high voltage to direct current with low voltage. Design of smart DC power source is currently greatest challenge for large scale application of electro-osmotic consolidation. Electro-osmotic consolidation is much quicker than preloading or vacuum consolidation for fine-grained soil with very low hydraulic permeability (Mitchell and Soga 2005). From point of view of energy consumption, faster consolidation requires faster energy input even though the total energy consumption is eventually the same (Zhuang 2016b); and faster energy input means larger power requirement. Therefore, field application of electro-osmotic consolidation normally requires high power supply. Power requirement depends on the volume of soil to be consolidated, type of soil, especially saline and non-saline soil, water content, etc. DC power source runs under low voltage, so high power means high current intensity. As a reference, consolidation of sludge of 1000 m2 area and 10 m depth, the initial current intensity can reach up to 5000–10000 A. Generally, saline soil with high ion content will have higher power demand than that of non-saline soil. It is common that consolidation of hydraulically filled area is carried out for thousands of square meters at the same time, so it is logical to consider the capacity of DC power source that can deal with 1000 m2 of area. There are two major strategies of DC power design. The first is to design a DC power source with high power; the second is to design and manage a lot of DC power sources with low power. Besides the strategies, feasibility of manufacturing under current technique also needs to be considered. Generally, DC power source with current intensity under 100 A is easy to build and that with current intensity over 1000 A will have large volume and weight, which is inacceptable in field application. In the field test presented in this paper the second strategy was adopted. This strategy was to build many DC power sources with power of 80 V/60 A. A central control system was needed for management of all the power sources. Roll polling program was adopted in the control system. The central control system and roll polling program is presented in detail as follows.


Roll polling program

Roll polling is a term borrowed from computer science. Roll polling electro-osmosis is a technique that allows the DC power source to carry out the electro-osmosis section by section alternately. This technique is based on the fact that dewatering will continue for a certain time after the input of electric current stops. Roll polling program is to make best utilization of electro-osmosis intermittent time (Zhuang 2016b). Roll polling program checked the voltage and current of every power source in turns in a certain time interval then adjusted the status of every power source. This was carried out by the sensor integrated in every power source of 80 V/60 A. It can record the current and voltage in the field automatically. Roll polling program was an essential program for the power source control system. In this power source, the roll polling program worked in the way as follows.

1) Although the total power in the field could be estimated, soil was inhomogeneous and it was difficult to estimate the current intensity in every position. Therefore, whatever voltage value was set the power source always started from low voltage, e.g. 2 V. Roll polling program scanned all power sources under 2 V, if there was power source whose output of current intensity was higher than 60 A, then this power source would be disconnected. When the total current intensity reached 400 A, no more power source was included in the connection.

2) If under 2 V there was any power source whose current intensity was smaller than 60 A but higher than 50 A, then all the power sources that were in connection status were set to work under 2 V.

3) Scanned the power sources in turns in interval of 1 min, if current intensity from all the power sources was smaller than 50 A, then increased the voltage in a step of 2 V.

4) Scanned those power sources which were disconnected in interval of 1 h, if current intensity for the power source was smaller than 60 A and total current intensity was smaller than 400 A, then included the power source into the connection status.

These are core function of roll polling program. Other functions are related to making best utilization of electro-osmosis intermittent time so that efficiency can be improved. This will be introduced in the following field test.