Page 53 - Energize August 2021

TECHNICAL

considerably on a short time scale. a full year, which means that such data must be available, a stumbling block for many sites.
The daily pattern of wind does not Daily variations in both wind and solar can affect the performance of the system, and detailed
lend itself to mathematical modelling and knowledge of not only the level of variation but how such variations combine and with what
analysis of complementarity of wind with regularity, is essential. The disadvantage of such a system is that a full years’ hourly data is
solar often requires an iterative process, required, and accurate and realistic data is often not available.
using high granularity data from short A simpler method would be to use average values and adapt the design to accommodate
intervals. Such data is often not available. possible variations. A factor which needs to be considered is the possibility of bad weather
Statistical analyses using long term days, which can affect both solar and wind systems. Reliable systems must cater for extended
average data is often used, but this does periods of low output from one of the resources. For solar this could be long periods of
not highlight the occurrences of extremely overcast weather, which can reduce power by more than 50%. For example, Gauteng and
low wind events outside the solar window, surrounding areas regularly experience up to seven consecutive days of overcast weather.
which are known to occur regularly, and This can be compensated for by additional storage or an additional resource not affected
could lead to the failure of the system. simultaneously by adverse conditions. Many of the projects mentioned in the literature have
found that the cheapest solution is solar PV plus storage, while adding wind to increase
Impact of storage on complementarity security also increases the unit cost.
The factor which has changed the hybrid The design process today could start with a single resource, wind or solar, and then go
sector more than anything else is the to a comparison of the relative cost of additional storage vs. a second resource to achieve
availability of low-cost, bulk transportable the required security of supply over extended bad weather periods. A common configuration
energy storage systems. These consist today in both smaller and large-scale systems is solar PV plus storage. The driver behind this
mainly of battery banks but could be is that the sun usually shines in most places, whereas wind can vary considerably depending
surpassed in the future by hydrogen- on site. The use of wind alone together with storage is uncommon.
based fuel cell electrolyser combinations,
or flow batteries. The decreasing cost of Utility scale hybrids
storage has made the use of more than A recent feature of the RE sector is the appearance of utility-scale hybrids. Large hybrid
one resource less attractive so that the systems, in the MW range, are already used for private generation, mainly on remote mines,
use of a single resource, generally solar but hybrid systems have recently appeared in the utility generation (i.e., grid connection)
plus storage, often constitutes the best sector. This seems to be due to much reduced storage costs and that dispatchability is
solution. The importance of low-cost becoming an important requirement of utility RE systems.
storage to the RE industry is illustrated There are a growing number of municipalities and other distributors planning to
by the fact that the US DoE has launched generate their own power from renewable sources or to buy directly from RE sources.
a programme aimed at reducing the cost Dispatchability is an essential requirement for these cases, and the hybrid system provides
of storage by 90% by 2030. If this goal is a solution.
achieved, it will change the whole face of Future networks based on a market system would favour hybrid RE systems which
the hybrid industry. could dispatch power when required. A despatchable system is the easiest to manage
and the most reliable. Most hybrid systems are required to deal with varying loads, which
Sizing and design could require complex control and management systems. The utility hybrid, at least for
Designs have been based on optimising the present, is required to deliver a fixed contracted load, which makes both design and
the balance between wind/solar and DG/ operation simpler.
storage set to make the maximum use The first utility hybrid system planned for South Africa is the Oya hybrid energy project,
of each component. There are a large wholly owned by G7 Renewable Energies (Pty) Ltd, a preferred bidder on the risk mitigation
number of protocols available to do this independent power producer procurement programme (RMIPPPP). The plant, which is to be
and most modern hybrids incorporate located near the town of Matjiesfontein, has a contracted maximum dispatchable capacity
energy management systems that apply of 128 MW, and will consist of 155 MW from PV, 83 MW from wind supported by a 160 MWh
an increasingly complex set of algorithms Li-ion battery. n
to control both the generation of energy
and consumption (demand management). References
There is no simple way of analysing 1. J Juraz, et al: “A review on the complementarity of renewable energy sources: Concept,
and designing hybrid systems, although metrics, application and future research directions”, Solar Energy 195 (2020).
some software packages are available, 2. M Handschy:“Is It Always Windy Somewhere? Occurrence of Low-Wind- Power Events
such as “Homer”, the Hybrid Optimisation over Large Areas”, Renewable Energy, Vol. 101, 2017.
Model for Electric Renewables. Most 3. S Jerez, et al: “Spatio-temporal complementarity between solar and wind power in the
processes work on a heuristic iterative Iberian Peninsula.”, Energy Procedia 40, 2013.
process that examines all possible 4. J Juraz: “The impact of complementarity on power supply reliability of small scale hybrid
combinations of components for cost, energy systems”, Energy, July 2018.
loss of power probability etc., and allows 5. A Naeem: “Maximizing the Economic Benefits of a Grid-Tied Microgrid Using Solar-Wind
the designer to select the combinations Complementarity”, Energies, January 2019.
that best suits the needs of the customer. 6. U.S. Department of Energy (DoE): “Hybrid Energy Systems: Opportunities for Coordinated
The process is based on an analysis Research”, 2021.
of hourly variables, such as wind and
solar data, as well as load information, for Send your comments to rogerl@nowmedia.co.za

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