IEEE Electrification Magazine - March 2015 - 14

TEchNOLOGY LEaDErS

Power (W)

50
40
30
20
10
0

1.5 3 4.5 6 7.5 9
Charging Time (h)

Figure 10. The charging characteristic of a
PBK.

400

300

200

100

0
0

40
8 12 16 20 24
Hour of Day

Bus Voltage (V)

Power (W)

and energy capabilities. In Muhuru
Bay, 10-20 PBKs are returned each day.
Because most of the customers have
students attending KCA, the PBKs are
usually returned in the early morning, coinciding with the start of the
school day. Customers are encouraged to return the PBK while there is
still appreciable charge remaining (as
indicated on the front panel of the
PBK) as this prolongs battery life and
decreases the time to recharge. Most
have a 40% state of charge when
returned. Based on these figures, a
reasonable estimate is that each PBK
will provide about 5-10 kWh per year,
which corresponds to Tier 1 electricity
access, as defined by the World Bank.
However, it is anticipated that the
energy consumption will grow as electronic devices proliferate throughout
the community.
Certain data from the kiosk are
automatically sampled and sent to a
server using the local cellular network. The data sent includes current

Figure 11. The average energy usage and
battery voltage daily profile for the energy
kiosk in Muhuru Bay, Kenya.

I E E E E l e c t r i f i cati o n M agaz ine / march 2015

from the PV panels and wind turbines; current to the diversion load;
DC bus voltage; inverter current, voltage, frequency, real power, and power
factor; cumulative energy into the station batteries; and control room temperature and humidity. The quantities
are sampled and transmitted each
minute using general packet radio
service, where they can be monitored
from anywhere in the world. Of particular interest in this installation is
the balance of energy. Over time, the
collected data will be used to determine if the kiosk can support additional PBKs.
The typical energy consumption
profile and dc bus voltage are shown
in Figure 11. The dc bus voltage, nominally 48 Vdc, is strongly influenced by
the production of power from the PV
panels and the load profile and to a
lesser extent the production from the
wind turbines. The coordination of the
diversion load and solar charge controller voltage set-points requires care.
When the station batteries approach a
fully charged state, the solar charge
controllers must disconnect before the
diversion load is connected to the dc
bus. This avoids a potentially hazardous condition in which the power
from both the wind turbines and PV
panels are diverted to the diversion
load, possibly overheating it.
The overnight consumption averages 200 W, mostly due to exterior
lighting and any PBKs left to
recharge overnight. There is a drop in
consumption around 8 a.m.-after
the sun has risen but before the
kiosk is open for business. The consumption fluctuates during the day
as PBKs are returned and recharged.
Consumption due to interior lighting
and small appliances is clearly seen
starting around 8 p.m. The load profile has changed over the course of
the few months that the energy
kiosk has been in operation and will
likely further evolve as PBK and local
usage patterns evolve. For example, a
refrigerator was recently purchased
so that the kiosk could sell cold soda,

7%
27%
33%
15%
18%
Energy Kiosk Structure
PBKs
Wind Turbines
Solar Panels
Microgrid Equipment
Figure 12. A breakdown of the capital
costs for the energy kiosk.

a highly desirable product in many
rural areas.
The energy kiosk and associated
components cost about US$37,000.
The cost per family is around
US$500, which is comparable to the
unsubsidized cost of a grid connection in Kenya. Figure 12 provides a
breakdown of the capital costs. PBKs
comprise a large portion of the
capital costs. They are also the components with shortest life span, typically lasting around two years. Each
month, about US$620 must be saved
to pay for the eventual replacement
of the PBKs and other equipment, as

2%
18%

80%

Maintenance
Salary
Equipment Replacement
Figure 13. A breakdown of the monthly
expenses for the energy kiosk.

Table of Contents for the Digital Edition of IEEE Electrification Magazine - March 2015