Meeting Energy Needs for a Home or Precinct like Chibobo

Student Task

Using the PDF document on the Reading Up On Renewable Enerygy page, and answer the following quesitons: Explain briefly each of the following terms: perfect (or peak) sun hours - PSH, PV array, inverter, battery regulator. What is the difference between a grid-connected system and a stand-alone system for PV arrays ? You should have worked out previously the total electricity demands of your house. (If unsure electricity bills for the year can be brought in and the maximum bill would be used to determine the energy used). From this information, the number of photovoltaic panels and the size of the battery bank can be designed or determined.

If your house were to be solely dependent on PV panels for its power determine the number of panels rated at 170 W required on your house to supply the power. The information below will help you to arrive at this. Various factors involved in the calculation (their explanation and typical values are listed below).

Ehouse	= Daily energy consumption (W.hrs) or the electricity used by the house
	= actual power (W) x hours of use (hrs)

In electricity, an appliance generally loses some of the power given to it due to its internal electrical properties. Thus in order for the appliance to work at the power stipulated on its specifications, known as the real power, a greater power called the actual power is supplied to it.

Actual Power =

Real Power Power Factor

The power factor is a number between 0 and 1. We will assume a power factor of 0.8 for all appliances. PF = 0.8

PSH = Peak sun hours (h, hours) – approximately 4 hrs for Melbourne

Cable loss (electrical energy lost as electricity travels through cables) = approx 5% (0.05) => cable efficiency = CL_eff = 0.95

Inverter loss (some energy lost in the inversion of electricity from DC to AC to suit the appliances) = approx 10% (0.10) and so the inverter efficiency = I_eff= 0.90

Photovoltaic Panel efficiency (the panels convert some electricity to heat) = approx 95% = PP_eff = 0.95

The temp. derating (where temperature causes photovoltaic panel inefficiency) eff = 87.5% => TD = 0.875

The dirt derating (where dirt on the panel prevents absorption of sunlight) eff = 95% => DD = 0.95

The battery banks may be inefficient in the storage of incoming electricity and lose some of the input energy => the battery bank efficiency = 90% => BB_eff = 0.90

As a general standard, photovoltaic panel numbers are designed to allow improved top up of battery banks = 110% => TU = 1.1

The panels have a maximum power rating => P_panel

Thus the number of panels required to provided the necessary energy per day is given as follows:

Number of panels required =

Ehouse x TU Ppanel x PPeff x PSH x CLeff x Ieff x TDeff x DDeff x BBeff x PF

Storage batteries are sized according to the unit amperes-hours, A.hrs. the relationship between energy in watt.hours and battery capacity in amperes.hours is as follows.

Battery Bank Capacity (A.hrs) =

Energy Supplied Used (W.hrs)

Battery Voltage (volts, V)

The battery bank needs to factor in the various inefficiencies along a part of the system as a result of energy losses in the passage of the electricity, and the number of autonomy days. This number (N_autodays ) represents a continuous number of days when there is little sunlight energy that is to be converted to electricity and so the battery has to be able to supply the house for a number of days without being topped up before it reaches a level of charge such that electricity can no longer be drawn from it. With lead-acid batteries the amount of charge drawn from them can not drop below 50%. If this occurs, the life and operation of the battery is reduced or compromised.

The lead-acid batteries operate at a voltage of 48V.

Battery Bank Capacity (A.hrs) = N_autodays x 2 x E_house(W.hrs) - assuming that lead-acid types are used.

V_batt x CL_eff x I_eff x BB_eff

Design of a Stand Alone Power Supply System (Chibobo orphanage)

(Please refer to the energy audit/power & energy summary of the Chibobo orphanage spreadsheet on the Chibobo Page)

Student Task

Use the spreadsheet to calculate the total power and energy usage of each appliance. You will need to sum the cells at the end of each row and at the base of each column for all the spreadsheet worksheets. The first worksheet (1-Power&EnergySummary) summarises the entire orphanage’s electricity use and the energy consumption has been worked out for the first column and first row. The 7 spreadsheet worksheets that follow show the usage of appliances per room or building. Which equipment consumes the most power? Which equipment consumes the most energy? How much energy is required for a day? What is the value of the energy required in the hour that the maximum energy is needed ? The following website has daylight hours information for Serenje. http://www.solar4power.com/solar-power-global-maps.html Use it to determine the typical number of peak-sun hours (PSH) for Serenje and thus Chibobo. Use the total energy audit information and the relationship below (as used previously) to determine the number of 170 W photovoltaic panels and the battery bank capacity needed at the Chibobo orphanage to supply the necessary energy. Use the two boxed relationships to calculate these quantities.

Website References:
http://www.bcse.org.au/default.asp?id=20
http://www.goingsolar.com.au
http://www.greenhouse.gov.au/renewable/pv/index.html
http://www.greenhouse.gov.au/renewable/home/index.html
http://www.greenhouse.gov.au/yourhome/technical/index.htm

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