paper 145 - Energy Efficiency of Cathodic Protection ...

ENERGY EFFICIENCY OF CATHODIC PROTECTION SYSTEMS R. Vukcevic, R. Brodribb, A. Pepe M. Brodribb Pty Ltd, Melbourne, Australia SUMMARY: Cathodic protection (CP) systems use electrical current from either a DC power source (Impressed Current) or from the preferential corrosion of anodes (Galvanic Current) to provide corrosion protection for metallic structures. Large CP systems may use energy sources of many kilowatts capacity to provide high currents for protection. The increasing cost of electricity, whether derived from the mains grid or from stand alone sources, such as diesel generators, requires designers to consider what will be the most economical energy solution for CP, when balanced with other performance parameters. This paper reviews and discusses the energy usage associated with the process of cathodic protection, including the energy used up in the transformer rectifiers, the input and output cabling and the anode circuits. The paper provides a comparison between a number of impressed current and galvanic anode CP systems, and assesses the energy usage and overall long term cost of ownership for the asset holder.

Keywords: Cathodic Protection, Energy Efficiency, Galvanic Anode, Impressed Current, Life Cycle. 1.

INTRODUCTION - ENERGY EFFICIENCY

Cathodic protection is an electrochemical method by which the natural corrosion of a metal, usually iron or steel, is inhibited by the supply of electrons from an external source to the metal. Iron is oxidized to give up electrons in an anodic reaction. This forms

Fe > Fe 2+ + 2e

(1)

The electrons produced in reaction then reduce water, oxygen or hydrogen at a cathode (metal surface)

2 H + + 2e > H 2

(2)

If an external source of electrons to the metal is provided, the anodic reaction of the iron is reduced and corrosion is prevented. Electrical current (the flow of electrons) must be delivered in such a way as to provide optimum corrosion protection, as insufficient protection will result in corrosion, and overprotection in both excessive energy use and possible damage to the structure. Electrons can be derived from passive sources such as sacrificial anodes using embedded electrochemical energy, or from active sources such as DC power supplies (CP rectifiers) which derive their power from external source, often an AC power grid. The metal surface does not distinguish between the sources of electrons. In either case energy is required to deliver the electrons to the metal and the energy efficiency of that delivery can vary widely. Energy is also required to build the structure that is protected, and if that structure has to replaced earlier than during it’s design life additional energy will be required to do this. Thus, we may consider that the infrastructure's total energy consists of a. the energy required to make the structure b. the energy required to make the corrosion protective devices, and c. the energy required to deliver the protective current.

Corrosion & Prevention 2012 Paper 145 - Page 1

1.1 Energy Efficiency of CP systems Simplified DC electrical models of impressed current CP (ICCP) and galvanic anode CP (GACP) systems are shown on Figures 1 and 2 respectively. The model for the ICCP system (Figure 1) is equivalent to a single cell battery charger, while the model for GACP system shown in Figure 2 is equivalent to a shorted battery circuit. Both models assume that the same amount of protective current is flowing into the structure surface, in order for the systems to be comparable.

Figure 1 Electrical model of ICCP system

Figure 2 Electrical model of GACP system

Current in ICCP systems is controlled by the CP rectifier and / or resistance RICCP, which represents all the resistive losses in the anode circuit. Voltage Vemf of the 'battery' represents all electrochemical potentials grouped together, as they develop on anode and structure surfaces. Voltage VCPR at the output of the CP rectifier is the active driving voltage in the circuit. Current in GACP systems is controlled by the Voltage VGACP and resistance RGACP, which represents all the resistive losses in the anode circuit. Voltage VGACP of the 'battery' represents active driving voltage, being a difference of electrochemical potentials between anode and structure. There are three levels on which the energy efficiency of CP systems can be observed and analysed: 1. on CP rectifier level (taking into account rectifier operation only), 2. on CP system level (taking into account system operation only), 3. on CP system level (considering both system construction and operation). 1.2

Power and Energy Efficiency

η

By electrical convention the Greek latter (Eta) is commonly used to signify the energy (or power) efficiency of a power system. It is defined as a ratio of the output over input power, or the output over input energy (both consumed over the same period of time) as per Eqn (1). Eta is expressed as a percentage and its value is always lower than 100%.

η=

Pout Eout = = [%] Pin Ein

(1)

However, while the CP systems are electrical power systems by nature, the main output parameter used in design of the CP systems is not power nor energy. Instead, it is the current density applied to the metal structure, which suppresses corrosion processes on the surface of the metal [11]. We suggest that a somewhat more generalised measure of the energy efficiency of CP systems be used. A different symbol could / should be used (Epsilon instead of Eta) to represent the CPEE, in order to distinguish the different nature of the energy efficiency ratio, as per Eqn (2).

ε=

Ein = [V ] I ×T

(2)

The CPEE ratio is not expressed as a dimensionless percentage, as it was in the case with symbol Eta, but it has a physical meaning being Joules over Coulombs, or Watts over Amperes, or simply Volts. The energy efficiency figure should be minimised, the lower the better, and CP system designers should strive to reduce the amount of Watts engaged per Ampere of the output current, or minimise the equivalent Volts of the CP system output.


2.

CP RECTIFIERS (RECTIFIER OPERATION ONLY)

With the rising cost of electricity, alternative types of cathodic protection rectifiers (CPRs) are being sought after and considered over the old Auto and Tapped Transformer rectifiers. Table 1. lists the three most common types of CPRs used. Table 1. General types of CPRs CPR Type

Power Efficiency @ 100% load

1

Tapped / Variable Transformer

~ 90%

2

Thyristor Controlled

> 80%

3

Switch Mode

> 90%

2.1 Transformer and Thyristor controlled rectifiers For all single power transformer based CPRs (Table 1. items 1 and 2) the main components that contribute to decreasing the efficiency are: 1. Transformer – Winding resistance, Eddy Currents and Hysteresis, 2. Silicon rectification components – High forward voltages, 3. Filter choke (If fitted) – Winding resistance, Eddy Currents and Hysteresis, 4. Filter capacitor (If fitted) – equivalent series resistance (ESR). These types of CPRs are of simple design which allows easy integration of additional monitoring features, but can have an operating power efficiency as low of 80%. 2.2 Switch-mode rectifiers For switch mode based CPRs (Table 1. item 3) the power losses are mostly located on the switching / rectification semiconductor components, and usually switch mode CPRs can operate at power efficiencies higher than 90%. The reason that the losses are lower is because the transformer size decreases with increase in switching frequency, resulting in the reduction of winding resistance, eddy current and hysteretic transformer losses. Switch mode CPRs are usually constructed with modular units working in parallel, and individual units can be turned on or off to optimize power efficiency during the low load periods [5]. 3.

CP SYSTEMS (SYSTEM OPERATION ONLY)

Representative models of ICCP and GACP systems are analysed, for a typical marine application requiring output current of 500A. While this current is chosen as an example of real life scenario, the subsequent calculation is largely independent of the current level (at higher currents range). We have assumed that the output current is constant throughout the protection period and that there is no variation in anode driving voltage. 3.1 Impressed Current CP Systems - traditional TRU example Electrical model of a typical ICCP TRU system with anode driving voltage of 10V is shown in the Figure 3. Energy efficiency is actually provided by the value of the output voltage of the CP rectifier divided by the power efficiency of the CP rectifier. For the example from Figure 3, where TRU CPR with power efficiency of 80% and 20V output voltage is used, the CPEE is ε = 25V . It has been shown that the resistance of the transmission cables for a high current system can greatly affect the initial setup costs and lifetime operating costs to the owners and operators [1,2,3,4]. These costs are derived from the resistance of the conductors over many hundreds of meters and the associated voltage drop. An underrated transmission conductor will reduce the initial installation cost, but will drive up the lifetime operating costs.


Figure 3 Electrical model of ICCP TRU system (single 100A channel shown) Table 2. is an example of a 500A 500m wharf with 5 anodes located every 100m and connected in a mesh arrangement. Table 2. Cost of power loss through conductors (Mesh) Conductor size (mm2) 185 240 300 400 500

Power Conductor current Resistance Power Cost Loss Per rating (Ω/km)2 Loss (W) (pa)3 1 Year (kW) (A) 530 0.1060 5,830 51,071 $10,829 630 0.0801 4,406 38,592 $8,183 730 0.0641 3,526 30,883 $6,549 850 0.0486 2,673 23,415 $4,965 980 0.0384 2,112 18,501 $3,923 1 Table 8 (Unenclosed touching) 110ºC operating temperature [10]. 2 Ignoring temperature correction factor. 3 Price of electricity at 0.21204 AUD/kWh as of June 2012 [6]. 4 Basic (no compound interest & no inflation) cost calculation of pa x 10.

Cost (10 yrs)4 $108,291 $81,831 $65,485 $49,650 $39,230

By observation of the Table 2. it can be seen that by spending slightly more on larger conductors during the installation can result in reduced yearly operating costs. The reduced energy loss in the conductors also allows for the CPR to be rated smaller and providing an extra source of saving. Table 3. is an example of a 500A 500m long wharf with individual supply conductors to 5 anode locations, which represents a traditional approach to ICCP system design. Table 3. Cost of power loss through conductors (Individual) Anode

Conductor sizes (mm2)

Lengths

Current rating (A)1

Resistance (Ω/km)2

Power Loss pa (kW)

0.78, 1.21

Power Loss (W) 1017

1

25, 16

45 + 55

145, 105

8,909

$1,889

2

50, 25

135 + 65

215, 145

0.386, 0.78

1028

9,005

$1,909

3

70, 25

260 + 40

270, 145

0.272, 0.78

1019

8,926

$1,893

4

95, 25

365 + 35

340, 145

0.206, 0.78

1025

8,979

$1,904

5

95

500

340

0.206

1030

9,023

$1,913

Cost (pa)3

Cost (10 yrs)4

$95,078

1

Table 8 (Unenclosed touching) 110ºC operating temperature [10]. 2 Ignoring temperature correction factor. 3 Price of electricity at 0.21204 AUD/kWh as of June 2012 [6]. 4 Basic (no compound interest & no inflation) cost calculation of pa x 10. Roughly 50% of the power of a CPR is dissipated through the conductors depending on the compromise made in the selection of the conductors and the expected lifetime operating cost. Corrosion & Prevention 2012 Paper 145 - Page 4

This then means that the required CPR output voltage has to be doubled to obtain a similar voltage across the anodes. 3.2 Impressed Current CP Systems - switchmode CPR example Electrical model of a switchmode based ICCP system is the same as for the TRU example, as shown on Figure 3 [5]. Using the same anode driving voltage of 10V and output voltage of 20V, and if TRU is replaced with an equivalent switchmode based CPR with power efficiency of 90%, the CPEE would be ε = 22V . 3.3 Impressed Current CP Systems - 'current multiplying' example Electrical model of a modular Current Multiplier ICCP system with anode driving voltage of 8V is shown in the Figure 4 [1,4].

Figure 4 Electrical model of ICCP CM system (single 100A channel shown) Energy efficiency is actually very closely defined by the value of the output voltage of the CP CM rectifier divided by the power efficiencies of PSU and the CM units [1,2,3]. By design, the losses in distribution cable are limited to less than 5% of the output power from current multiplier. For the example in Figure 4, where output voltage is 8V and respective power efficiencies are 90%, the CPEE would be ε ≈ 10V . 3.4 Galvanic anode CP systems - example Electrical model of a standard GACP system is shown in the Figure 2. We assume the same protective current of 500A, while the anode driving voltages will depend on the anode material used. It should be noted that anodes are not made of pure metals, but of alloys containing about 5-10% of other elements [12]. Relevant embodied energy content ranges for dominant anode materials can be seen in Figure 5 reproduced from [8], and further values are cited in [7]. For the purposes of this analysis the assumption is that the embodied energy for the anode will not differ by more than ±5% compared to the value for the pure dominant metal. The energy efficiency of a GACP system can be expressed as ratio of embodied energy of the anode material (in MJ/kg) over the anode capacity (in Ah/kg). Respective CPEE values for aluminium, zinc and magnesium anodes would be ε = 34V , ε = 36V and ε = 113V , using the values quoted in Table 4. 3.5 CP systems - Comparison Table 4. summarises the CP system examples presented above. Analysis was carried out as if the CP systems were operating at 100% capacity during the protection period, ignoring the initial polarisation phase of the structure and variations in the output that may occur afterwards. While the model is relatively simplified, it does provide interesting insights. The first finding is that ICCP systems can be generally more efficient than GACP systems - by a factor of between 1.3 and 3. However, it is worth mentioning that for ICCP systems with output voltages higher than 25V there is no comparative energy efficiency advantage over GACP systems, as the CPEE for them rises above 32 Volts.


Figure 5 Embodied energy vs. cost for metals, from [8] The second finding is that aluminium and zinc anode systems have almost identical CPEE, with aluminium having far superior weight ratio. Further, magnesium anode systems tend to be less efficient, by a factor of about 3, compared to other two GACP systems, based on data available. Third insight is that innovation in approach to ICCP system architecture can bring superior CPEE figures, particularly if very low anode driving voltages are adopted (5V or less). Table 4. Energy Efficiency comparison of various CP systems Impressed Current CP Galvanic Anode CP Switch Current TRU Aluminium Zinc Magnesium unit of measure mode Multiplier 500 500 500 500 500 500 A 20 20 20 20 20 20 years 20 20 8 V 80% 90% 75% % 12.5 11.1 5.3 kW Cu Cu Cu Al Zn Mg 2,550 780 1,230 Ah/kg 80% 80% 80% % 5,000 5,000 1,000 42,941 140,385 89,024 kg 571 571 571 2261 721 ~4002 MJ/kg 1 1 1 1 1 17 17 17 25 9 MJ/kg 57 57 57 251 81 400 MJ/kg 79 79 16 2,994 3,159 9,892 MWh 2,190 1,947 934 MWh 2,269 2,026 950 2,994 3,159 9,892 MWh 26 23 11 34 36 113 V (W/A or J/C)

CP System total current protection period CPR output voltage CPR power efficiency input power (AC) material (anodes, cables) anode capacity anode utilisation factor weight (anodes, cables) embod. energy virgin embod. energy recycled embodied energy applied total embodied energy total consumed energy total energy energy efficiency (CPEE) 1

Values taken from [7].

2

Value taken from [8] (Figure 5).


The entry for "weight (anodes, cables)" includes the weight of copper cables (for ICCP systems only) They are included to make the comparison to GACP systems more balanced, as cables represent probably the largest embodied energy component in ICCP systems. The entry for "embodied energy applied" for copper cables takes into account only the virgin figure, instead of the more realistic combination of the virgin and recycled, to illustrate that even if the exaggerated energy value is used, it does not contribute much in the overall calculation. This is because the total embodied energy for cables is comparatively small as it is amortised over the protection period. The entry for "embodied energy applied" for anodes is a sum of virgin and recycled energy values for anode materials, as usually the anode manufacturing process has a recast step in forming the anode specific shapes from appropriate alloy. The sum is further justified by the fact that in the process of cathodic protection the anodes are corroded away, and therefore their material is lost (i.e. the original primary energy spent in production of virgin material is lost) and can not be recovered through recycling. The entry for "anode utilisation factor" is taken as an average expected value. Different anode materials behave differently when exposed to environmental factors, and anode utilisation factor can range from 50 to around 90%. This means that not all anode capacity is converted into usable electric charge (i.e. protective electric current), as there are some losses to selfcorrosion of the anodes, or the consumption of metal on the anode surface may not be evenly distributed [12]. The Volt as unit of measure for CPEE is quite interesting when viewed in the context of results from the Table 4. The values for ICCP systems correspond to normalised voltage outputs as if CP rectifiers did not have any internal losses. We can than view CPEE values for GACP systems as their 'equivalent' voltage outputs (as compared to the actual driving voltages in GACP circuits which are usually in a range 0.2V to 1.5V). 4.

CP SYSTEMS (SYSTEM CONSTRUCTION AND OPERATION)

Currently, there is no commonly accepted definition for what a CP System is, nor what are the basic system elements. For our purposes, the widest possible definition should include all materials, assemblies and devices that are installed for and used to support the function of the cathodic protection. 4.1 CP System Elements CP system can include all or some of the elements listed below: - Anodes (impressed current, galvanic), - Cables (for anodes and bonding networks, AC delivery, reference electrodes, etc.), - Cable support systems and mounting hardware, - CP rectifiers, - Monitoring and Control systems (reference electrodes, interrupters, remote monitoring electronics and modems, etc.), - Energy supply components (transformer stations, switch boards, etc.), - Surge protection systems (lightning strike rods, earth stakes and mats, surge diverters, etc.), - Enclosures (buildings, shelters, cubicles, etc.), - Air-conditioners, etc. All of these elements contain embodied energy, and some will consume energy (and dissipate it as heat) during the operation of the CP system. 4.2 CP systems In the light of recent endeavours to view infrastructure assets over the entire life cycle [9], the energy efficiency can be generalised even further. In addition to the energy spent during just the operation of the CP system, all other energy inputs can be taken into account to calculate the total or life cycle energy efficiency. The sum of all of the energy factors should include energy used for: 1. production of materials and structures used in system construction (embodied energy), 2. transport of materials and structures to and handling on site, 3. CP system construction, 4. operation the CP system (including testing and commissioning), 5. maintenance and repair of the CP system and its elements, 6. site clean-up and disposal of the CP system after decommissioning. As a prerequisite for prevention of the need for an excessive power capacity of the CP system, the Life cycle energy efficiency should be calculated after the Corrosion Protection of the structure is optimised, (i.e. some or all of the following steps are already investigated and implemented). This would be most important on the newly built structures: Corrosion & Prevention 2012 Paper 145 - Page 7

- areas of bare metal are covered, i.e.: * ship hulls are painted, * pipelines are coated and insulated, * water tanks are coated, etc. - physical protection methods for the structure are applied: * coating of concrete surfaces (to reduce chloride ingress), * splash zone mechanical protection (waves, ice, floating objects, etc.) - use of corrosion resistant materials in construction where possible: * stainless steels, aluminium, titanium, etc. * plastic, etc. Ultimately, there is a super Life cycle energy efficiency level for the whole structure, which is sitting above the LCEE for just the CP system. This super-LCEE should take the above listed corrosion protection measures into account. 5.

CONCLUSIONS

The rising cost of electricity and newly introduced carbon tax is pushing the corrosion market in a more energy efficient direction to reduce the overall lifetime operating costs of the system. This may then require older systems to undergo a new cost analysis to determine whether they need a CPR replaced with a more efficient model and or conductor sizes increased. It is a valid exercise to analyse CP systems energy efficiency on three separate levels: On existing structures - for CP rectifier retrofits power efficiency of new CP rectifier units should be an important factor of choice. On existing structures - for complete CP system retrofits or new CP system installation for the first time, the CPEE of the entire system should be considered. Finally, for newly built structures, the LCEE of the entire structure should be addressed, within which the CPEE for the CP system should be optimised. In any of the above cases, the energy efficiency optimisation can contribute significantly to reducing the energy consumption during the construction and operation of CP systems. Aluminium and zinc galvanic anode CP systems are shown to have almost identical CPEE, with magnesium anode systems trailing behind the other two by being less efficient by a factor of about 3. ICCP systems are found to be able to be more efficient than GACP systems - by a factor as high as 3. The exception is for ICCP systems with output voltages higher than 25V, for which there is no comparative energy efficiency advantage over GACP systems. Innovative approaches to ICCP system architecture are capable of having superior CPEE figures to all other CP configurations (both ICCP an GACP), particularly if anode driving voltages of 5V or less are implemented. 6.

ACKNOWLEDGMENTS

The authors are grateful to the environmental engineer Joan Ko for providing the references to information and data on embodied energy for metals. 7.

REFERENCES

1. Vukcevic R, A Novel 'Green' Approach to Powering Marine ICCP Systems, Proc. Corrosion & Prevention 2008, Nov 16-19, 2008 Wellington, New Zealand, Paper 129 2. Vukcevic R, Godson I, Improving Anode Loop Energy Efficiency of Marine ICCP Systems, Proc. Corrosion & Prevention 2009, Nov 15-18, 2009 Coffs Harbour, Australia, Paper 68 3. Vukcevic R, Godson I, Furstenberg J, Resolving CP Design Restrictions at an Australian Wharf: A Novel Marine Impressed Current CP System, Proc. Corrosion 2010, Mar 14-18, 2010 San Antonio - Texas, USA, Paper 10031 4. Vukcevic R, Furstenberg J, Marine ICCP Systems Based on Current Multiplier Technology Materials Performance (NACE) 49(3) (2010), 30-34 5. Brodribb R, Czarski J, Design and Implementation of a Modular High Current Power Supply for Cathodic Protection use, Proc. Corrosion Control 007, November 2007, Sydney, Australia, Paper 38


6. National Greenhouse Accounts Factors, Appendix 5, Electricity emission factors for end users, 1990–2010, pp 62-69, Australian Government, Department of Climate Change and Energy Efficiency, July 2011 7. Hammond G, Jones C, ICE Inventory of carbon and energy ICE V2.0, (Excel version of the database) Sustainable Energy Research Team (SERT), Department of Mechanical Engineering, University of Bath UK, Jan 2011 8. Metals energy cost diagram, Cambridge University Materials Group in Department of Engineering, Cambridge, UK, (www-materials.eng.cam.ac.uk website last accessed July 2012). 9. PAS 2050-2011 - Specification for the assessment of the life cycle greenhouse gas emissions of goods and services, BSI (British Standards Institution), 2011 10. AS/NZS 3008.1.1:2009 Electrical installations - Selection of cables - Cables for alternating voltages up to and including 0.6/1 kV - Typical Australian installation conditions, Australian Standard, 2009 11. AS 2832.3-2005 Cathodic protection of metals – Fixed immersed structures, Australian Standard, 2005 12. von Baeckmann W, Schwenk W, Prinz W, Handbook of Cathodic Corrosion Protection: Theory and Practice of Electrochemical Protection Processes, 3rd Edition, Gulf Professional Publishing (Elsevier Science), USA, 1997 8.

AUTHOR DETAILS Richard Brodribb is the Managing Director of M. Brodribb Pty Ltd. He has been involved with the design and manufacture of a wide range of DC power supplies for cathodic protection, battery charging and other DC applications for over 30 years. He has a B.E from Monash (Elec Eng) and a M.B.A from RMIT. He is a member of the IEEE, NACE International and ACA.

RAJKO VUKCEVIC is a Design Engineer at M. Brodribb Pty Ltd. He has over 30 years of electronics design experience in switch-mode power applications and 10 years of impressed current CP engineering experience in Australia. He is involved in design and development of electronics systems and electronics equipment for ICCP used in marine, concrete and pipeline applications. He is an inventor of the Current Multiplier based Power Distribution System for marine ICCP applications. He has a M.S. (Elec Eng) from Belgrade University. He is a member of ACA and NACE International.

ARI PEPE has been an Engineer at M. Brodribb Pty Ltd for the last 4 years after finishing university. He is involved in design and development of electronics systems and electronics equipment. He is a member of ACA and Engineers Australia.
