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Solar: A power(full) alternative for the cold chain

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An example of Solar-powered cold rooms located in East and Southern Africa. Image credit: Ag Funder

An example of Solar-powered cold rooms located in East and Southern Africa. Image credit: Ag Funder

By Hannes Enslin, Technical Product Manager at Auto X (Pty) Ltd; editing and introduction by Benjamin Brits 

With the continual rise in conventional energy costs and slow electricity grid expansion in South Africa, this power source is not only a convenient stand alone alternative, but importantly a green solution.

As businesses find different ways to solve their power needs, and in many cases lack of service in outlying areas – and the dreaded load shedding – solar power is in fact a very suitable and reliable option that, like most systems you would work with today, comes in a variety of options – from full stand alone to grid-tied.

The cold chain is a particular element where solar power can be a solution and especially in bringing cold stores closer to the farmers or producers without extreme measures to install the required power infrastructure in remote areas.

In many instances, solar products have the reputation of being ‘expensive’, but as technology and demand has grown over recent years, costs generally have come down, and play on an equal par with conventional and alternate supply, and in some cases is a better and cheaper overall solution.
With future developments, this technology as well as its green aspect will likely be a significant contributor to the growing energy demand as global growth of this energy solution is expected to exceed 10% this year (estimates pre-Covid-19 were at 14%).

Components that form part of a solar system

A solar power or solar energy generation system consists of the following main sub-systems/components:

  • Photovoltaic (PV) panels (or commonly referred to as solar panels). These convert solar radiation (photons – light energy) into electrical energy (coulombs).
  • Solar charge controller (SCC) – This component regulates the PV voltage and power to match the inverter or battery energy storage system (BESS) direct current (DC) bus voltage and input power requirements.
  • Inverter – An inverter converts the DC energy from the SCC or ESS to alternating current (AC) to power the AC loads.
  • Battery energy storage system (BESS) – A battery bank for storing surplus energy for later use.
  • Balance of plant (BOP) – DC and AC protection (breakers, fuses, transfer switches), solar panel mounting-structures, earthing and lightning protection components.
  • Monitoring and control system

Many solar inverters nowadays have built-in solar charge controllers, as well as monitoring and control functions to enable grid-synchronisation. Quite often they also have an AC battery charger built-in to provide for charging of the batteries from the grid or a generator when solar energy is insufficient – this is referred to as a hybrid inverter.

It is estimated that nett-zero emissions can be achieved within 4 years of the plant or system installation.

Distribution warehouses with rooftop solar panels installed at one of the many Amazon facilities in the US. Image credit: Amazon

Distribution warehouses with rooftop solar panels installed at one of the many Amazon facilities in the US. Image credit: Amazon

Types of solar power systems

Solar system terminology can be confusing, or often used incorrectly causing confusion, but the main categories are as follows:

Grid tied system: Inverters are connected and synchronised to the utility grid and must switch-off when the grid fails. ‘Tied’ to the grid and not allowed to function in ‘island’ or off-grid mode. Typically used in large PV power generation plants or larger roof-top PV systems for self-consumption generation, these systems reduce the energy bill and potential maximum demand fee, and export excess energy into the grid if allowed by the service provider or the municipality. Worldwide this type of system is used from small residential applications to a large utility scale.

Off-grid system: The system is suitable where no utility grid power is available or the inverters and the AC loads it serves are not connected to a grid. These are typically used in rural and remote areas and usually also are installed with a larger BESS to provide for longer autonomy (back-up power) to compensate for any bad weather.

Grid-assisted system: This is effectively an off-grid system in the sense that the inverter provides power to the load in a ‘UPS’ mode and the load circuit is isolated from the grid supply. The inverter can however supplement power from the grid and/or charge batteries from the grid when solar energy is insufficient. The system can operate in ‘island’ mode to provide power to the load during grid failures and load shedding. Some inverters can also export excess energy to the grid, again if allowed. This system is used in the majority of smaller scale domestic and commercial applications for security of supply during grid failures/load shedding and reducing energy bills.

Solar power systems and their suitability to the cold chain

As an alternative energy source for refrigeration/cold storage, investment in a solar energy system can contribute both in terms of security of supply (provide power to priority loads) during grid failures/load shedding, as well as reducing the electricity bill and maximum demand charges.

This applies to any electric energy consumer including the refrigeration/cold storage sector, where it can also provide savings on generator fuel consumption and maintenance costs due to the criticality to maintain refrigeration/cooling during grid outages to prevent/reduce product spoiling and losses.

The same applies in remote/rural areas where grid reliability is poor, and becoming worse, and/or where service providers do not plan capacity extension, or the cost of the expansion outweighs the return on investment of own (solar) generation to meet business expansion needs.

There are ‘micro-grid’ and renewable energy models on the market that are internationally accepted by the World Bank, US Aid, Power Africa, and the EU, that can be used to design system concepts and trade-off analysis with bankable results.

For the logistics sector, solar panels or flexible solar panels can theoretically be fitted on refrigerated transport trucks, trailers and container roofs. Limited surface area/generation capacity, potential material damage and theft should to be taken under consideration when determining the feasibility for use in this application.

An onboard BESS with an inverter/charger that can be recharged at depots, fuel stations or even potentially at EV charging bays along the national road network is definitively something that we may see in future as overseas developers investigate ways to make this a reality. We already have vast developments in electric vehicles that too would make use of this system.

The most important considerations in sizing a solar system

The most important parameter is always money or access to funds: What can you afford to invest (CAPEX) and what is your expected break even term and hence potential ROI. One would need to consider how much the investment will reduce your annual:

  1. energy usage/consumption bill
  2. maximum demand bill
  3. (diesel) generator (DG) fuel and maintenance costs
  4. other losses directly linked to load shedding/grid failures such as production loss, scrap, rework, etc.

It is important to understand your business and what the split between critical and non-critical loads are and what the power (kVA/kW peak and average) and energy (kWh) requirements are for the critical loads.

The power requirement will determine your inverter size and the energy BESS capacity. The BESS is usually the most expensive sub-system and could be traded-off against you existing back-up generator OPEX cost and/or CAPX and OPEX for installing a new or additional DG(s).

Graph showing installed PV capacity trend for SA 2019.

Graph showing installed PV capacity trend for SA 2019.

Incorporating solar with conventional electricity supply

Solar systems can be incorporated with existing electricity supply system with minor changes to the existing AC distribution board(s), connections and reticulation – depending on the current set-up and intended installation, and of course in accordance with national and local regulations.

Solar power compared to conventional and other alternate solutions

Even with the expected electricity tariff increases, solar electricity at this point in time remains a high CAPEX investment and careful consideration of expected break-even period and potential ROI remains important.

The initial CAPEX cost of a PV system is much larger than that of a DG or gas generator, but there is definitively a break-even point where after the PV will pay for itself. A PV system has very little OPEX whereas a DG/gas generator has large OPEX in terms of fuel consumption, maintenance as well as major overhauls or repairs.

Depending on the type of cold storage technology, a refrigerated coolant reservoir can also act as energy storage system.

Wind generation is an attractive addition and could potentially generate power 24/7 and not only when the suns shines, but most regions in South Africa do not have adequate constant wind conditions for wind generators, while the local climate is said to be a perfect setting for solar power with long days and lots of sunshine for generation.

Key elements system designers should keep in mind

In addition to those already mentioned, roof space, roof weight loading capacity, roof orientation and/or carports or land area for erecting the PV arrays and potential shading are all important considerations.

Many factors influence the expected yield from a rooftop solar system, but for an example for estimation purposes a north-facing 30 degree rooftop with 100m² under solar panels in an area with good annual sunshine – the result could provide an average year-round generation of about 75kWh per day.

It is also very important to decide if your design is based on a once-off installation or a phased implementation approach – expanding capacity later. This will determine the inverter selection and system design as well as initial AC connection and AC DB/reticulation changes to ensure you can expand later and avoid unnecessary duplicate costs.
There are also tax benefits and potential carbon tax reductions to add into the cost/benefit analysis when considering solar power as a green alternative.

International PV Levelised Cost of Energy (LCOE) cost trends where the same trend applies to South Africa.

International PV Levelised Cost of Energy (LCOE) cost trends where the same trend applies to South Africa.

Solar and carbon footprint

In terms of total carbon footprint, solar energy has of course a zero carbon footprint in generation. However, keep in mind that all the equipment and material required for the system are mined, processed, manufactured, and transported – so, a complete solar system doesn’t have a zero carbon footprint.

Technology developments in recent years

In addition to the continued development in the electronics industry, both inverters and solar panels have moved towards higher DC voltages of 1 000V to 1 500Vdc per PV string to reduce system losses, improve efficiency and reduce balance of plant cost.

Even hybrid inverters are moving to high voltage battery DC bus voltages to match the PV DC input side to reduce inverter cost and battery cable costs.

Solar cell material and efficiencies, as well as module (panel) construction are also steadily improving with mono-crystalline panels topping a 20% efficient under standard test conditions (STC). Bi-facial panels are also becoming popular for solar plant/carport installations.