Applicability of solar PV for rooftops

July 10th, 2015, Published in Articles: Energize

 

There is a lot of chatter about whether to put “this solar thing” on rooftops. Often, however, the full impact of such an intervention is not understood, and so the differences between the solar options should first be clarified. Solar thermal applications are energy saving (reduction of electricity consumption) where radiation is converted to usable heat through a liquid medium, most often water. Examples of this are solar geysers and absorption chilling.

Photovoltaic (PV) solar power, as its name suggests, is an application where sunlight is turned to electricity. It is part of a group of interventions known as “embedded generation”, where the energy generator (PV system) is embedded in the user’s own electricity network.

Fig. 1a: Type 1: constant load facility, solar overlay.

Fig. 1a: Type 1: constant load facility, solar overlay.

 

Fig. 1b: Type 1: constant load facility, applicability.

Fig. 1b: Type 1: constant load facility, applicability.

This article focuses on the applicability of grid tied solar PV for various building types, in very general terms, and PV experts should be consulted for finer details when sizing the solution and determining which financial indicators will be applicable for your specific building. The analyses presented here are for conceptual understanding of the impact of installing a PV system on a particular building type.

Building energy consumption (load) profiles and monthly bills vary significantly from building to building, and are based on a number of key factors, including kWh consumed, peak kVA (maximum demand), time of use, seasonality, critical loads and where the power is used (common areas vs. private or specialised areas, for example). While no two buildings are exactly alike, it is likely that the building in question falls into one of four main categories:

  • Type 1 has a fairly constant load throughout the day and night. An example would be a manufacturing facility.
  • Type 2 is the bunny ear model, seen often in gyms or homes. This model often has a large morning and evening peak, and sometimes a lunchtime peak too.
  • Type 3 shall be referred to as the office building, which has a permanent base load, but then a peak which generally follows office hours.
  • Type 4 is a mixed use profile, which is made up of combination Type 2 and 3 loads. This would be seen, for example, in a mall where there may be a gym and bakery operating early and late in the day, with office loads during the middle of the day.
Fig. 2a: Type 2: solar overlay.

Fig. 2a: Type 2: solar overlay.

 

 

Fig. 2b: Type 2: solar overlay with feed-in tariff.

Fig. 2b: Type 2: solar overlay with feed-in tariff.

When we overlay a day’s solar output over this, we can determine both the maximum size of the system we should use, as well as an indication of what the net result will be on the remaining demand from utility or electricity supplier.

It can be clearly seen that in constant load facilities, a correctly sized solar system can be very effective and is a good fit. It is important to note that financial models should include kWh savings, but that demand reduction, if any, is generally not a factor, and should therefore not be part of the business case.

In Type 2 buildings, it is important then to either size the system small enough so that the maximum amount of PV energy produced is less than the daytime load, alternatively the regulatory framework must allow for feed in tariffs or net metering. Any financial model must then use this information, or be aware that any power produced during daytime hours, which is not consumed, must either be fed into the grid, or be wasted by limiting the active power output of the inverter. This is the model that would generally be used for grid-tied homes – in South Africa we do not yet have a national or standard municipal framework which allows for this in all areas, although some pilot studies are underway.

Fig. 3a: Type 3: office building solar overlay.

Fig. 3a: Type 3: office building solar overlay.

 

 

Fig. 3b: Type 3: office building applicability.

Fig. 3b: Type 3: office building applicability.

In office buildings, or other daytime use buildings, there are definite kWh savings, but also the potential for kVA (peak demand) savings. While it is important to be aware of all the factors contributing to solar production and the building peak demand (including such things as HVAC systems usage profiles, cloud cover and weekend occupancy levels) empirical data are now showing that there are definite reductions in peak demand on some installations. Some of these clients also find that the demand savings are not always predictable on a daily, weekly or monthly level due to the many factors involved as well as seasonality of these factors, but that on an annual basis there are very measurable impacts.

It is interesting to see that in mixed use buildings, the impact of a solar PV installation results in the return of a Type 2, or “bunny ear” model.  This is due to the fact that the solar profile follows very closely on the office hours people work, and therefore eliminates that portion of the load. Only limited demand savings are seen here, but there are certainly other measures that can be used to generate peak reductions – the applicability of technologies will be based on the devices used to actually create these loads (switching to gas cooking for restaurants, or load shifting using storage are examples of how this could be achieved). Although this paper does not cover eco buildings solutions, many such papers exist on these topics.

Finally, as a conclusion to the applicability of solar solutions to various building types, it is prudent to also explain the factors which determine the feasibility of any grid tied solar project.

Fig. 4a: Type 4: mixed use buildings solar overlay.

Fig. 4a: Type 4: mixed use buildings solar overlay.

 

 

Fig. 4b: Type 4: mixed use buildings applicability.

Fig. 4b: Type 4: mixed use buildings applicability.

There are at least three key factors, which should always be considered when purchasing a PV system:

  • Capital cost: this is the actual price you pay for the solution. While often this is viewed as the defining consideration, it would be a terrible mistake to rely on this as the main driver. That would be like comparing a second hand car with no warranty to a brand new one. The running costs and performance of the car are what actually determine the overall value of the purchase – not just the price of the vehicle.
  • Operating costs: these are the ongoing costs which need to be provided for to ensure the systems keeps producing the required results. Factors here include ease of service and therefore cost of service, warranty value (does the warranty actually exist, or will the company who sold it to you be out of business in four years should you have to claim for something), availability of service (if something happens to the initial installer, is there someone else who can still support the solution). Generally the actual maintenance costs of PV installations are very low when compared to other plant solutions as the only moving parts in the system are the cooling fans and the balance of maintenance is around ensuring the PV modules (solar panels) are kept clean.
  • System yield: this is the actual amount of energy that the PV system actually produces. So many times an RFP/Q is released where the customer asks for a price for a 200 kW solution (for example), and the evaluation criteria then points immediately to price. In fact, there are so many different possibilities for a 200 kW system (does the client need 200 kVA, is it 200 kWp, what level of module oversizing is required or preferable) – in most cases this is simply because the client does not understand enough about the technology yet to make an accurate request. In keeping with the analogy, they end up purchasing a 2ℓ car because it was 2%cheaper without understanding that the competitor had in fact offered them the 2ℓ turbo diesel motor of the same car, with the same output at the coast, but a better performance on the Highveld due to the turbo reducing the effects of altitude on performance. The system yield is then made up of two factors – conversion efficiency, and harvest efficiency [1].

Once the applicability of the PV system is confirmed, the procurement process should then evaluate capital expense, running costs and system output (yield) to ensure the value of the solution meets expectations of both the client and installer/solutions provider. For those who are not yet sure of the procurement process themselves, there are various funding models in place where the funders have done all the necessary due diligence on the systems suppliers and installers already, and assume the technology and performance risk themselves, although with an investment return attached.

Finally, it is also important to understand the regulations around connecting to a public network of any sort, whether LV or MV. There are different rules and avenues depending on whether the supply is directly from Eskom or a municipality – please always ensure that all necessary advice and permissions are in place before beginning a project, certainly for regulatory reasons, but primarily as a matter of safety for anyone who may have access to the electrical infrastructure in or beyond an embedded generation zone.

Reference

[1]    Dr. A Swingler: “PV String Inverters and Shade Tolerant MPPT: Toward Optimal Harvest Efficiency and Maximum ROI”, www.schneider-electric.co.za/documents/support/white-papers/seshadetolerantwp.pdf

Contact Ntombi Mhangwani, Schneider Electric, 011 254-6400, ntombi.mhangwani@schneider-electric.com

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