Thursday, 5 July 2018

NEWSLETTER #27 - SOCIETY OF AFRICAN EARTH SCIENTISTS








Volume 7, Issue 1

January-March 2018


CONTENT
Chair's Foreword
Renewable Energy Battery Regulation and Recovery in off-grid PV Systems
Earth Science Events
References and selected reading



Chair's Foreword*
This issue addresses the problem of renewable energy battery life as applied to Africa where batteries are a very expensive  yet very vulnerable component of  off-grid photo-voltaic  (PV) solar electricity energy systems.

Renewable Energy Battery Regulation and Recovery in Off-grid PV Systems

Introduction
The regulation of battery charge when rechargeable batteries are used to store energy drawn from solar panels (or wind or hydro-turbines), is crucial if battery life is to be maximised and this expensive component of the electric system is to be sustained.
   It is worthy of note that there are social as well as environmental issues that impact on the effectiveness and even economic feasibility of this technology. The fear of theft, for instance,  can sometimes restrict options for users in where they feel able to locate solar panels.  Also there may be limitations on roof  space which is not shaded and therefore open to sunlight. The structure of roofing may be such that direct sunlight is only received by the panels  for part of the day. These factors do lead to the battery/battery bank not being fully charged on a regular basis, which is important to sustain battery life.
   Although the price of solar energy is rapidly falling, especially in comparison to fossil fuels, it still remains a prohibitively expensive technology, out of the reach of many of the African population in terms of affordability.   It therefore is of great interest to ascertain if system batteries that become deeply discharged, for one reason or another, can be recovered; saving the relatively unaffordable cost of replacement, as well as the prospective environmental hazard associated with battery disposal.


Basic Considerations in Battery Charging
In a photo-voltaic (PV) electrical energy system with

  • a battery /battery bank which stores energy collected from the solar panels (or wind or hydro-turbines)
  • a power inverter to convert DC current to AC current for household appliances
  • a charge controller to regulate the charging of the battery/battery bank from the solar panels (or other source) so the battery does not become over-charged (which can release toxic gases) or discharged below 80% of its capacity,
the charge controller will cut out the inverter and stop the system working once the battery charge drops to 50%. Contrary to what might be expected, battery charge state is only inversely related to the battery voltage above approximately 21 volts, for a 24 volt battery/battery bank, as the graph below illustrates (figure 1.). At 100% charge the voltage of a 24v battery/battery bank is 25.5 volts. At 90% charge it is 25.24 volts; at 50% it is 24.20 volts; at 10% it is 23.02 volts, and so on.

Figure1. Percentage % Battery Charge Remaining for a 24 volt Battery/Battery Bank

   In a PV system the charge controller would normally prevent the batteries from discharging below  80%  of capacity. However, if the PV system is disconnected for some period, the batteries will slowly discharge in storage and if left without recharging for a considerable period, they will become very deeply discharged. The discharge is more severe if  the battery/battery bank remains  connected to equipment, such as the power inverter, or even the charge controller itself.
   From basic principles, we know that if voltage is applied to the battery system , which is greater than the battery's voltage, a current will flow through the battery (in reverse direction to when it is supplying a current) and will charge. The rate of charge or current that will flow will depend on the difference between the battery voltage and that of the energy source for recharging (e.g., solar panels).
   Common wisdom until recently was that such a battery would be beyond recovery and effectively dead. However, battery techhnology is under the spotlight and there is a view that slow charging with solar panels can recover deeply discharged batteries; which should be welcome news for solar energy system users in the developing world.
  In the following considerations we focus on the lead/acid battery, which would appear to be the most commonly used type of rechargeable battery supporting PV systems in Africa. The lead Acid battery  illustrated in figure 2, consists of positive and negative terminals both made of lead plates. As the battery charges,  the process of electrolysis delivers a layer of lead oxide to the positive terminal; whilst discharging the battery causes the lead dioxide  and lead on  the plates to be  converted to a soft lead sulphate. The reaction dilutes the acid and makes it less dense.

Figure 2. Lead/Acid Battery with positive and negative lead plate  terminals
     
Lead - Acid Battery Regulation and Recovery


Diaz and Egido (2003) in their paper on battery charge regulation [1], make technical recommendations for the charge regulation of lead/acid batteries. They note that batteries are the weakest component in the PV system since they rarely perform at full capacity and often diminish in storage capacity with time. This diminution is mainly due to failure to maintain full charging of the battery, for reasons including

  • randomness of solar radiation
  • personal consumption pattern of each PV user
  • cost and availability
  • battery working in limited range due to various factors, limiting the expected long battery life. 
Diaz and Egido discover from their tests, significant variations in  the discharge profiles of batteries depending on the make of charge controller; indicating that choice of controller is an important factor in ensuring the optimum regulation and operation of the battery.
   The authors recommend maximum depths of battery discharge which they say should not be exceeded.
   In practice, circumstances as already described do mean that these depths are often unavoidably exceeded for one reason or another. However, earlier work of  Spier and Kasinkoski (1995) goes to the trouble of testing if deep discharging really does mean the fatality of the battery [2], and not simply accepting the common wisdom, which has not been sufficiently tested. These earlier authors writing in the Journal of Power Sources, posit the crucial and exact question that needs to be answered :
"Can the battery recover from a very deep discharge at a low discharge rate, followed by a slow recharge under PV-type conditions?"[3]  The authors answer this by concluding that in general most batteries can be recovered whilst losing no more than about 10% of their charge capacity, after storage in a deeply discharged condition.
   This result means that we can avoid massive wastage of solar battery resources  provided we are able to recharge in the correct manner. Apparently, many standard battery chargers are not always able to fully recharge deeply discharged batteries, because a safety mechanism  prevents this. However, a safe and  effective method is to follow-up the use of a battery charger with the application of a slow charge via solar panels, which may take days, but will deliver battery charge back to almost full capacity.



Earth Science Events

April 17-19, 2018

The 7th Digital Earth Summit

VISION: In the decades since the coining of the concept of Digital Earth, we have seen technological advances in earth observation, Geographical Information Systems, communication networks, grid computation and other areas of globally oriented digital technology. The International Society of Digital Earth (ISDE) has worked in collaboration with Chouaib Doukkali University, the African Association of Remote Sensing in the environment  and the Moroccan Association of Remote Sensing in the environment to host a conference on Digital Earth for sustainable development that brings together scientists and other professionals from Africa and the international community to present their achievements in research, share expertise and experience.
VENUE: Faculty of Science, Chouaib Douakkali University,
El Jadida, Morocco.


July 14-15, 2018

IASTEM, 420th International Conference on Environment and Natural Science

Lagos, Nigeria.


July 21-28, 2018
27th Colloquium of African Geology and 17th Conference of the Geological Society of Africa
VISION:This is a major biennial meeting organised this year by Aveiro University and Geological Society of Africa, under the auspices of Geological Society of Africa.  The meeting invites senior and early career scientists from all over the globe to participate and  foster international cooperation.
VENUE: Aveiro University, Portugal.

July 22-23, 2018

IASTEM, 425th International Conference on Environment and Natural Science

VISION: Scholars, scientists, engineers and students present and share their ongoing research activities with a view to enhancing research relations globally.
VENUE: Port Luis, Mauritius.

August 30, 2018

IASTEM, 449th International Conference on Environment and Natural Science

VISION: Scholars, scientists, engineers and students present and share their ongoing research activities with a view to enhancing research relations globally.
VENUE: Lagos, Nigeria.

October 24-25, 2018

ISERD, 475th International Conference on Environment and Natural Science

VISION: Scholars, scientists, engineers and students present and share their ongoing research activities with a view to enhancing research relations globally.
VENUE: Cape Town, South Africa.



October 25-29, 2018

The 12th Conference of the African Association of Remote Sensing of the Environment

VISION: A conference focused on earth observation and geospatial science in service of sustainable development goals. Conference themes include 1) Big data and data mining of geospatial data. 2) Climate change implications for sustainable development. 3) Geospatial science for early warning systems for geohazards.4) Influence of African space policy on the youth generation 5) Remote sensing for natural resources management, etc.
VENUE:Alexandria, Egypt.



References and selected reading

  1. Diaz, P. and Egido, M.A., Experimental Analysis of Battery Charge Regulation in Photovoltaic Systems, Prog. Photovolt: Res. Appl. 2003; 11:481-493 (DOI: 10.1002/pip.509).
  2. Spiers, D.J, and  Rasinkoski, D, Predicting the service lifetime of lead/acid batteries in photovoltaic systems, Journal of Power Sources 53 (1995) 245-253.
  3. Ibid., p. 248.
*Board of the Society of African Earth Scientists: Dr Chukwunyere Kamalu (Chair - Nigeria), Ndivhuwo Cecilia Mukosi (South Africa), Osmin Callis (Secretary - Guyana/Nigeria), Mathada Humphrey (South Africa), Dr Enas Ahmed (Egypt).



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