Page 100 - A Comprehensive Guide to Solar Energy Systems

Chapter 5 • Sustainable Solar Energy Collection and Storage 95

module [36], totalling €205.32 for 600 Wp, €345.15 for a 250 A h lead-acid battery (based on
commercially available batteries in rSA), and €65 for a charge controller and cables. This
gives a total initial system cost of €615. Over a period of 20 years, given the typical number
of cycles from a lead-acid battery, the battery would need to be replaced roughly once
every 4 years (5 batteries in total), bringing the 20 year cost of the system to €1996. Fig. 5.10
shows the cost of the system over time, including these battery changes and the value of
the electricity produced, assuming all costs remain constant. What may be surprising to
those working in PV is that most of the system cost is associated with the batteries, that is,
with the storage of energy rather than its generation. The need to reduce system costs to
create cost-effective and affordable PV solutions for rural Africa should be noted.

5.6 Energy Storage—Battery Choices

Four main battery technologies dominate stationary energy storage applications
( Table 5.2) [38]. lithium ion (li-ion) batteries represent the majority of installed storage
capacity, and are commonly used in domestic photovoltaic systems. The merits of lead-
acid batteries for applications in rural Africa have already been mentioned in describing
their use in the Swansea University Zambia project. Vanadium redox flow batteries (VrFB)
require pumps for electrolyte flow and additional energy and storage capacity to support
this. This, along with the additional mechanical complexity of VrFB systems, makes them
unsuitable for this small-scale application. High temperature NaNiCl batteries are also
unsuitable because of the hazards associated with molten metal electrodes.
Cost is of paramount importance for this application, and Table 5.3 compares costs for
three candidate technologies for the proposed system: valve regulated lead-acid (VrlA),
li-ion, and Aquion saltwater batteries. lead-acid batteries are significantly cheaper than
lithium-iron-yttrium-phosphate (lFyP), or Aquion, batteries (∼1/10th and 1/5th the cost
respectively). The easy availability and low capital investment costs of lead-acid batteries
are very attractive, but lead-acid has relatively low cycle lives compared with li-ion and
Aquion batteries, and a high sensitivity to deep discharge. Significant oversizing of capac-
ity is therefore required. The relatively short lifetimes for lead-acid batteries mean that
these must be replaced 4 times over the lifetime of the proposed system, resulting in a total
cost of €1726, which is still only one-quarter the price of the longer lifetime lFyP (€7104)
or one-third that of Aquion (€5040) batteries.
Of course, economies of scale and direct bulk purchase from manufacturers may result
in lower battery prices. The use of a circular economy approach can also give cost savings
by: using remanufactured, refurbished, or repurposed batteries; purchasing batteries
manufactured from recovered materials; and valorising end-of-life batteries to recoup
costs by diverting them to refurbishment, remanufacturing, and recycling. In addition,
opportunities which enhance the longevity of batteries should be explored, to reduce
the number of necessary replacements over the system lifetime. Such opportunities for
batteries in South Africa are discussed later.