by Conrad Roedern
Following the booming development in many countries, grid-connected Solar PV systems owned and operated by electricity end-users are also becoming of interest in Namibia. The Renewable Energy Industry Association of Namibia – REIAoN is in favour of this development, and recognises the importance of having clear and workable procedures for the accounting of electricity produced by endusers. This paper does not deal with Independent Power Producers (IPPs) who sell electricity generated from renewable resources to a national buying entity, or possibly an electricity distribution entity. Rather, this paper focuses solely on so-called embedded Solar PV generators, i. e. small-scale generation capacity at or close to the premises of an end-user. During past workshops held by the Electricity Control Board (ECB) net metering has been identified as the preferred accounting method for embedded PV generators.
Net metering in practise
In its most elementary form, net metering means that “the electricity meter of an electricity end-user can turn both ways: increasing the metered kWh units if electricity is drawn from the grid, and decreasing the metered kWh units if electricity is fed into the grid”. Net metering is one of the ways in which end-users who produce some of their own electricity from embedded generation capacity can account for their net use of electricity from the grid.
Clients add with their generation system – in the terminology of the engineer – a “negative load” to their system in order to compensate a meaningful share of their traditional “positive loads” for which they have to buy electricity. Some of our “positive loads” have a detrimental effect on the grid since they can cause flicker and harmonics (e.g. from rectifiers, thyristor controls, appliances with high inrush currents) or phase shifts between voltage and current due to a bad power factor ( e. g. from motors and other inductive and capacitive devices) while the “negative loads” comprising of a Solar PV array and a special grid-tie inverter counters the detrimental effects of “dirty” loads to some degree.
Net metering allows the importing of electricity from the distribution network, whereby the end-user purchases energy from the grid, as well as the export of electricity to the grid, whereby the end-user feeds electricity into the grid. In this way the grid acts as a battery.
Payment for net metering services is simple: The end-user still pays for a monthly basic connection fee, as well as (in some cases) a circuit breaker fee, plus the net electricity that is drawn from the grid. Here, net electricity drawn from the grid is defined as “the difference between electricity imported minus electricity exported”. If, as may be the case occasionally, the net electricity drawn from the grid is negative, then the end-user has exported more electricity to the grid than what was imported. In such cases, the end-user would not have to pay for any electricity used in that month, and would be remunerated for the net amount of electricity fed into the grid at the end of a pre-defined accounting period. For the grid-operating utility, this implies that they receive injections of green electricity, which contribute to the ability of the grid to supply additional units of electricity to other consumers, while stabilising the grid during the day and in times of mid-day peak demand.
Past workshops held under the auspices of the Electricity Control Board (ECB) have identified net metering as the preferred method for small-scale embedded generation having a capacity of less than 500 kW.
Net metering guidelines proposed by REIAoN:
- An annual billing cycle is followed, whereby the accounting period of the meter balance takes place over one year.
- There is no cash pay-out from the grid operator if the trading balance for a period of less than the accounting period shows a net export to the grid; as per point 1 this will only take place once per year.
- Embedded systems larger than 500 kW will require a license by the ECB, unless excluded under the Namibian Electricity Act of 2007 and its Regulations.
Requirements for quality and safe operation:
- Standard safety precautions (NRS97) for programming the compulsory anti-islanding device of the grid-feed inverter(s).
- Grid-feed inverters have to set standards in terms of safety, and in terms of parameters including voltage limits, frequency limits, harmonics, power factor and time before automatically disconnecting from the grid. Certified grid-feed inverters are industry-standard since a long time.
Advantages of net metered PV systems:
Net metering is simple, using conventional electricity metering technology as used in Namibia. The end-user contributes to the grid operator’s revenue through connection and other fees, and thus continues to pay for the services provided for and through the grid. The end-user is afforded an opportunity to positively contribute to Namibia’s critical power supply situation. It creates an incentive to produce power at the point of consumption while the investment in generation capacity is on the end-user’s account and does not have to be borne by NamPower and/or the Government of Namibia. A win-win situation is created whereby the end-user and grid operator have advantages from such additional locally available embedded generation capacity. It reduces line losses, especially in the lowand medium voltage reticulation systems and reduces electricity purchases from outside Namibia, which is an especially critical aspect if regional electricity markets are constrained, as is currently the case. It affords traditionally passive consumers a chance to become productive. No special feed-in tariff is required in many cases. Especially in situations of industrial use, generation and consumption follow a similar pattern and thus cancel each other, which is particularly lucrative for the grid operator who sells additional electricity made available on the grid through embedded generation for the productive use of industrial and manufacturing clients. No separate power purchase agreement (PPA) is required, which reduces the bureaucratic burden on the grid operator and end-user operating an embedded generation plant. Clean green electricity with high environmental value is added to the system at a reasonable cost: It can swiftly increase the highly desired green electricity share in the grid towards the Kyoto target
On the technical side, the systems inject extremely clean sinusoidal power with low distortion (T.H.D.) and power factor. Roof-top systems can have considerable extra benefits like providing extra shading for a roof (reduced cooling load) or even serving as roof on a carport. The country benefits since feed-in systems are designed and manufactured locally; thus their local content is much higher than of any other generation option.
Net metering does not incentivise owners of embedded generation capacity as is for example done under specialised “feed-in laws” such as Germany’s “Renewable Energy Priority Act (EEG)”, where PV system operators are enabled to sell all their produced electricity for a cost-reflective price fixed for a 20-year period. However, net metering is a most practical and simple to implement method for allowing end-users to become active participants in Namibia’s electricity distribution sector, and in this way positively and economically contribute to the country’s electricity supply crisis. As such, net metering should be adopted as the standard for all grid-connected renewable energy providers below 500 kW capacity. It constitutes a win-win situation between the grid operator and the end-user owning and investing in embedded generation capacity, and balances their respective interests. Net metering is practised in many countries around the world, and while helping to fill the current generation gap, it also actively supports the much-needed increase of “green” electricity contributions in Namibia’s electricity mix.