Energy for green buildings: Options and opportunities

Energy savings on basic architectural design, lighting systems, climate control and building machinery and appliance management, to reduce building energy demands, have greatly reduced the energy load of buildings. After all this there is has been done there is  still an energy demand to be met, consisting of electricity and heat, and this is the subject of green building energy supply.

The definition of a green building encompasses a range of subjects (measures) all aimed at reducing the environmental impact of a building, and has resulted in a system of building ratings. The ratings are mainly applied to commercial  buildings,but also to others such as medical facilities, and campuses. There are a number of measures used to rate the “greenness of a building”, such a zero waste buildings, zero water buildings etc. but this article will focus on the energy consumption measures.

Fig. 1: Hybrid CSP solar PV/Th  system (Cogenera).

Building energy ratings

There are a number of classifications for energy efficient buildings, some of which overlap depending on country or organisation. Assuming that all measures relating to energy efficiency and energy savings have been dealt with, there still remains a residual requirement for electricity and heat, which can be met in several different ways according to the rating desired to be achieved. The aim of green building energy is to reduce the net energy consumption, as well as reducing the carbon footprint of the building.

Some definitions


Generally seen as a single structure or a number of structures owned and operated by the same entity, on a defined site, and may include undeveloped ground on the site owned by the entity. Includes parking spaces and open storage yards.

The carbon footprint of a building 

This is generally taken to the amount of non-carbon-neutral “greenhouse” gases, mainly carbon dioxide, emitted as a result of activities undertaken within the building bounds. This is not restricted to emissions on site but includes those related to the production of items consumed on site but produced elsewhere such as fossil fuel based electricity. Emissions on site do not include CO2 and other GHGs produced by the occupants of the building which are carbon neutral. Carbon neutral GHGs are produced from biomass which is an atmospheric carbon sink. The use of biomass or waste on site to produce heat or electricity is carbon neutral but may cause particulate air pollution.

Net energy consumption

This is the balance between energy drawn from an external source such the electric grid, gas or liquid fuel network, and energy generated on site. The balance need not be instantaneous or in real time, but may be taken over a time period.

Green building energy ratings

Zero net energy (ZNE) building

At the heart of the ZEB concept is the idea that buildings can meet all their energy requirements from low-cost, locally available, non-polluting, renewable sources. At the strictest level, a ZEB generates enough renewable energy on site to equal or exceed its annual energy use. The building may import more energy than what is generated at various times, but the annual balance must be zero or positive. On-site generation need not be renewable, but this is the general aim. A ZNE building remains connected to the grid A building which is highly energy-efficient, and the remaining energy use is from renewable energy, preferably on-site but also off-site where absolutely necessary, so that there are zero net carbon emissions on an annual basis (net zero), or if the energy from renewable energy results in more energy being produced than what is used on site (net positive).

Fig. 2: Flat panel hybrid PV/T module (Solimpleks).

Zero net carbon building (ZNC) [1]

This is a building that is highly energy-efficient, and the remaining energy use is from renewable energy, preferably on-site but also off-site where absolutely necessary, so that there are zero net carbon emissions on an annual basis (net zero), or if the energy from renewable energy results in more energy being produced than what is used on site (Net Positive) [1]. A ZNC building remains connected to the grid. A building that is highly energy-efficient, and the remaining energy use is from renewable energy,preferably on-site but also off-site where absolutely necessary, so that there are zero net carbon emissions on an annual basis (net zero), or if the energy from renewable energy results in more energy being produced than what is used on site (net positive).

The ZNC rating is not necessarily a zero energy building, as renewable energy or carbon-free energy (e.g. nuclear) may be imported from the grid or from a remote site or IPP to achieve a carbon free balance for the site. The site itself may not necessarily be zero carbon but this may be offset by carbon sinks elsewhere, an approach which is not favoured but allowed.  Imported renewable hydrogen from a gas type network is a future possible consideration which will also allow ZNC operation without being ZNE compliant.

A future scenario, where there is a large amount of renewable energy available on the grid, offers the prospect of achieving a ZNC rating with very little on-site RE generation. A recent American study [2] claimed that the cost of rooftop or on-site solar PV is double that of utility scale grid solar PV. If this is the case it should, in the future, be cheaper to purchase renewable energy from the grid than to generate on site, and the concept of ZNC building may become superfluous, although the ZNE concept may still be useful( as a status symbol) if somewhat more costly than the grid supply option.

All South African buildings currently consume a small amount of renewable electricity from the IPPs supplying the grid, but this varies as their output varies, and as the IPP production figures are not made public, it would be difficult to keep track of this.

Zero emissions building (ZEB)

An emissions-based ZEB produces at least as much emissions-free renewable energy as it uses from emissions-producing energy sources. An on-site emission ZEB offsets its emissions by using on-site renewable sources. If an all-electric building obtains all its electricity from an off-site zero emissions source (such as hydro, nuclear, or large scale wind farms), it is already zero emissions and does not have to generate any on-site renewable energy to offset emissions. However, if the same building uses natural gas for heating, then it will need to generate and export enough emissions-free renewable energy to offset the emissions from the natural gas use. Purchasing emissions offsets from other sources would be considered an off-site zero emissions building.

Options for on site zero-carbon energy production

Building energy requirements are for electricity and heat. Electricity for lighting, environmental control and building as well as commercial activities. Heat for water heating and climate control. Heat  supply is normally derived from electricity, but with ZCE can be derived from other sources.

Sources of on site zero carbon emission generation:

  • Solar PV
  • Solar thermal
  • Wind
  • Waste (biogas and biomass)
  • Biomass (wood and wood pellets)
  • Fuel cells

Solar PV

If we ignore items such as BIPV and solar roof tiles which are low efficiency and  are not generally used, solar PV is the most common on-site renewable generation technology used in commercial and industrial buildings. It has the advantage that existing roof structures and shade structures (parking) can be used. It is also simple to install and to integrate with the electricity reticulation system.

Fig. 3: Large PV/T installation (Solimpeks).

Solar thermal

Solar thermal generation is commonly used in South Africa for supplying hot water, although there are several high temperature solar thermal (HTST) systems in operation as well. HTST can be used to provide energy for cooling using adsorption chillers. STG is more efficient than solar PV with a conversion factor of up to 80%, depending on conditions.


Wind is not commonly used for onsite power generation in buildings, although several large factories have installed MW sized wind turbines on site. This may change with the development of vertical axis wind turbines, (VAWT) which are quieter than horizontal axis types and can handle the turbulence and gusting experienced in the vicinity of buildings better.

Waste (biogas and biomass)

Small scale biogas production is usually restricted to agricultural sites, remote lodges and industrial sites with a high waste load  is unlikely to find application in a general commercial buildings in this country. Production of energy from biomass waste, either by direct combustion or gasification,  requires a high waste load and is unlikely to find application in general commercial buildings except where boilers for heating are already installed in buildings.

Biomass (wood pellets and wood)

Generation of electricity and heat from waste or carbon neutral sources such as wood or wood pellets is not generally used in South African buildings. Besides which wood product and waste combustion causes air pollution and is not desirable.

Fuel cells

The fuel cell has the advantage of producing both heat and electricity simultaneously with a much smaller physical footprint than solar PV or wind. Large fuel cells (>5 MW) have been installed at institutes such as hospitals and campuses. Currently run on natural gas or LPG and have a lower carbon footprint than grid electricity when heat generation is taken into account. The use of natural gas means that they are not green but a lower carbon source than coal based grid electricity. The Fuel cells primary fuel is hydrogen, and if and when “green” hydrogen from renewable energy sources, such as surplus wind production becomes available, fuel cells could be considered as a very promising  “carbon free” source of electricity and heat for green buildings.

There is one building in South Africa using fuel cells at the moment [3], but this installation runs on gas so is not carbon free. The use of fuel cells based on “renewable” hydrogen fuels to produce both heat and electricity is not well enough developed in this country to consider at the moment. Considering all the above methods, the only practical means of generating on-site renewable energy would be wind and solar.

Analysis of options

Wind technology, although well developed, is not generally used in on site power generation for buildings, and is subject to wide variations in wind speeds, and will not be considered. The ability of a building solar PV/thermal energy system to generate sufficient energy to balance the energy consumption of the building depends on the energy consumption per m2 of building floor space, the total floor space (TFS) of the building, and the total roof and other space available on the building ( for convenience this total is referred to as solar mounting space (SMS) in this article .

A useful ratio to consider is the ratio of solar mounting space to total floor space or roof to floor space ratio ( R/F ratio). For a single storey building with a simple roof the ratio would be approximately 1, and will always be <1, if only the roof space is used. For multi storey buildings the ratio will be at most 0,5 and will decrease as the number of storeys increases, with a decreasing ability of the solar RE system to meet or exceed building energy requirements.

Solar energy capabilities

The efficiency of solar PV panels varies with the technology. Taking the most common technology, Multicrystaline silicon, at an efficiency of 15%, a well installed solar installation will produce 150 W/m2 at standard insolation conditions. (A typical commercially available panel is rated at 300 W for 2 m2 of panel surface) .Taking an average annual insolation to be 5 kWh/kWp per day for major South African cities, solar PV can be expected to produce:

1826 kWh/kWp = 1826 x 0,15 kWh/m2/annum = 273 kWh/m2/annum                   (1)

In the simple analysis the building will achieve ZNC status if the annual energy consumption multiplied by the total floor space is less than 273 multiplied by the SMS. Or the energy consumption is less than 273 x R:F ratio. For example if the R/F ratio is 0,25 then the energy consumption must be less than 68,25 kWh/m2/annum.  In the case of a building with additional parking or storage space in addition to roof space, this must be added to the roof space in calculating the feasibility of the project.

Building loads

Building loads in South African buildings can vary widely with the seasons and with the type of building. Target values for different types of energy efficient buildings as set out in SANS 204 –Section A – Energy usage in buildings, and other documents are given in table 3m for buildings with artificial environmental control [2, 3].Values vary with the climatic zone, and figures in Table1 are for zone 1, which includes Johannesburg, Bloemfontein and Vereeniging.

Table 1:  Maximum energy consumption in buildings with artificial environmental Control [3].
Building type Targeted maximum annual energy consumption kWh/m2/annum
Sans 204 JHB City guidelines for new buildings
Entertainment and public assembly 420 420
Theatrical and indoor sport 420 420
Education 420 420 ( 120 CIBSE)
Places of worship 120
Large shops (malls) 240 240
Offices 200 200
Hotels 650 650 (340 for guest houses and B&B)
Hospitals and clinics 650
Residential)  all types < 40
Light industrial 120
Heavy industrial 240

Table 2 gives the maximum energy density requirements for buildings with  natural ventilation[3]. This classification covers smaller building for which natural ventilation is practical.

 Table 2: Maximum energy consumption for naturally ventilated buildings [3].
Building type Target energy intensity (kWh/m2/annum)
Office 171
Retail 152
Clinic 211
School 209
Residential 210
Category 1 house 298

Meeting ZCB ratings requirements with solar PV

From Table 2, using the value of 273 kWh/m2/annum from eqn. (1), it can be seen that only single shopping malls, places of worship and office’s will be able to meet the ZEB or ZNC requirement by using roof mounted solar PV. If the SMS is increased by using parking or storage yard space to mount solar PV, then the R/F ratio may be increased and most single story buildings would be able to meet the requirements. Very few multi-storey buildings would be able to meet the ZCB requirement, even using parking and yard space. The most suitable building would be single storey building with a low energy usage density. Single storey shopping malls (which also have large parking areas) and warehouses fall ideally into this category.

This result can be compared to figures for other countries such as the average of all US office buildings, which is 250 kWh/m²/annum. According to EU passive designs, a building is deemed energy efficient if its uses <120 kWh/m²/annum. In Australia the almost equivalent climate zone the almost equivalent climate Zone 4 has a warm dry summer/coldish winter – there the base building BCA compliant value for a medium sized office is 243 kWh/m²/annum. It is of interest to see that Greek buildings have an impressive average annual consumption value of 135 kWh/m²/annum, because they traditionally don’t make use of air conditioners for cooling [2].

Now while none of these figures are strictly comparable, it seems the SANS 204 expectations are certainly ambitious for a country accustomed to a cheap and endless supply of electrical power. Solar PV only works during sunlight hours and electricity consumption outside of PV production hours will have to come from grid electricity or from storage of electricity generated during the PVP period. Electricity storage for building is in its early days and is not generally used, neither will it be for the immediate future, so the grid is generally used as the energy balancing mechanism, either by means of a feed-in or net metering agreement. The arrangements for net metering in South Africa vary from distributor to distributor, and the principle is not favoured by many distributors, making on site storage of electricity the option of choice for the future. On site storage also ensures improves security of supply.

Solar heat generation

In most buildings Solar thermal generation is used for production of hot water for washing and food preparation. STG is more efficient than PV, Typical systems used for hot water production consist of flat plate collectors or vacuum tube collectors. In many existing buildings, hot water is provided using electrical geysers, and this is included in the total electrical load. Heat is also used for comfort environmental control in winter months and this adds to the total electrical load on the building. The heat needs for buildings such as hostels, hotels and sport stadiums where actual showering and washing takes place are much higher than office and general purpose buildings.

Most heat requirements can be met by STGs, but the use of STGs reduces the space available for solar PV, and the offset in electricity consumption must be offset by reduced electricity generation. If the electric heating cycle is taken into account, and all efficiencies are measured, then it may make sense to replace electric heating with solar heating. A careful analysis needs to be done on both heat and electricity demand before allocating roof space between PV and STG systems.

A practical problem is that STG only works during the day, and evening consumption will have to be offset by additional PV production during the day. Thermal demand may be estimated using the figures in Table 4, which  gives the annual  hot water heating power densities specified in SANS 204.

Table 4: Heating power density for different building types [2].
Building type Heating power density (W/m2)
Residential 9,61
Hotels and guest houses 4,65
Hospitals 20
Offices 0,23
Retail 0,14

Hybrid solar PV/solar thermal system

A development which has important consequences for the green building applications concept is the hybrid solar PV/thermal unit, which produces both electricity and heat, using the same mounting space. A typical system uses a trough type concentrator with both a PC and a thermal collector at the focus, as shown in Fig.1 .Other systems use flat panel designs with the thermal collector mounted below the PV collectors.

The system installed by Cogenera at Sonoma wine company has a capacity of 222 kWth and 50 kWe [4]. Other types of  PV/T system use flat panels with a thermal collector behind the PV cells (Fig. 4).  A flat panel example produces 200 We and 600 Wth. The solar thermal output of a PV/T hybrid is two to three times the electrical output and no additional mounting space is required. A further variation mounts the PV cells inside an evacuated tube thermal solar collector. The flat hybrid PV/T system has been installed at numerous sites around the world in sizes ranging from domestic installations to large institutions.


[1] Green building council south Africa: Green star tools

[2] M Barker: “SANS204 – Energy efficiency in buildings”, Urban energy support.

[3] SANS 204 Part A: Energy usage in buildings, 2011.

[4] P Davis: “Solar Cogeneration: Introduction & Overview”, Solar thermal 11.

[5] F Yazdanifard:  “Investigating the performance of a water-based photovoltaic/thermal (PV/T) collector in laminar and turbulent flow regime”, Renewable Energy 99.

[6] “Design Guidelines for Energy Efficient Buildings in Johannesburg”, City of Johannesburg.

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