6.1 Direct absorption of solar thermal energy
Using South Africa, as a case study, with an average of about 6.2 kWh/m2 per day of usable thermal energy (as per Fig. 1) and a 3x3.5 m or 10 m2 collection area system on a house rooftop, up to 50 kWh of energy per day, or about 1200 kWh per month (assuming 75% sunny days per month), can be collected on the rooftop of a SA household. In this way, a good thermal absorber fitted on rooftops of SA households, could basically supply most of the energy needs of a medium scaled household in SA, especially if this energy could be converted to other forms of energy.
Our group at UNISA CSET has subsequently developed a novel thermal energy absorber system of 1x 3 m2 dimension using an arrangement of low cost polyethylene piping as illustrated in Fig. 6 (a) and (b). The piping system is placed on a reflective plate using a standard corrugated plate structure that is normally used in the construction of roofing for SA households. The piping is then arranged in a specific configuration in order to increase the absorption per unit area for a 23-degree incident angle of the sun as prevalent in South Africa. Water is circulated through the system in a series parallel manner in order to maximise energy absorption at lower temperatures. Thick piping were used in our second iteration prototyping in order for the absorber to simultaneously serve a s a storage of thermal energy. Nanotechnology designed films are used to increase the absorption reduce loss of thermal heat at high temperatures The circulated water is then fed by means of a small booster pump into a standard geyser. The system acts as a “preheater” of water, implying that the thermal element in the geyser is only used to provide top up heating of the water in the geyser to a specified temperature Several versions of the system have already been developed. Tests show that about 10 kWhr can be absorbed by the system withing 4 hours of exposure in the sun. The system has recently been extensively patented by UNISA CSET ( RSA patent 05297, 2016).
Table 3: Cost-efficiencies versus energy-densities of various common energy supplying commodities for the SA household environment (Google, 2023)
Gas/Compound/item
(energy-density decending)
|
Energy Density
(MJoules/kg)
(kWh/m3)
(Alterantively Specified)
|
Cost
(ZAR per kg )
|
Green sustainable Index
(0-10)
|
Advantageous of apllication/ comments on sustainabilty/ reference
|
Natural Gas
|
33 – 82 (10.6)
|
50
|
2
|
To be contained in cylinder /explosive hazard/polluting gas
|
Liquid Pretoleum Gas
|
46 (12.8)
|
40
|
3
|
To be contained in cylinder /explosive hazard/ clean gas
|
Methane gas
|
50
|
50
|
3
|
To be contained in cylinder /explosive hazard/ atmosphere harming gas
|
Hydrogen gas
|
120
|
100
|
7
|
To be contained in cylinder /explosive hazard
|
Wood
|
20
|
5
|
5
|
Resaonable Safe/ esaily containable/ poluting gas/selfsustaiable
|
Coal
|
24
|
10
|
3
|
Resaonable Safe/ esaily containable/ polluting gas
|
Crude Oil
|
44
|
10
|
3
|
Polluting gas
|
Petroleum
|
35 - 45
|
2
|
3
|
Resaonable Safe/ esaily containable/ polluting gas
|
Wind
|
500 W/ m2 at 10 km/h
|
|
9
|
Safe, little envirnmenenatl impact
|
Compressed Air
|
0.1 per m3
|
10
|
9
|
Explosive hazard
|
Peltier Heat Cell
|
0. 00004
|
500
|
9
|
Resaonable Safe/ esaily containable/ no pollutants/ recycable
|
Photo Voltaic Cell
|
.000003 (0.1)
|
500
|
9
|
Resaonable Safe/ esaily containable/ no pollutants/ recycable/ cnon polluting over longer term
|
|
|
|
|
|
|
Key to Table:3:
Cost/energy/sustainabilty (normalised)
|
Applicabilty Colour Index as allocated
|
0-3
|
|
3-5
|
|
5-7
|
|
7-10
|
|
Table 4: Energy storage options for the SA household environment (Google, 2023)
Gas/Compound/item
|
Energy Density
(MJoules/kg)
(kWh/m3)
|
Cost
(ZAR per kg )
|
Green sustainable Index
(0-10)
|
Advantaegous apllication/ sustainabilty/Disadvantgeous
|
Lead acid battery
|
0.0024 (2.4 )
|
10
|
7
|
Resaonable Safe/ esaily containable/ no pollutants/ recycable
|
Lithium Ion battery
|
0.5 -0.7
|
5000
|
7
|
Resaonable Safe/ esaily containable/ no pollutants/ recycable
|
Compressed Air
|
|
|
|
|
Water
/kg/degC)
|
0. 0042
|
0.02
|
10
|
Safe/ esaily containable /non polltable/selsustaiable
|
If a thermal absorber system of the above dimensions could be manufactured and installed on SA households for about R10 000 with a lifetime of twenty years, it imply that the cost of a thermal absorber system could supply needed energy (for example hot water ) at about 0.35 ZAR per kWh. This is then a most viable form of energy that can be harvested or utilised in the SA household environment, according to our current research.
6.2 Conversion of stored thermal energy to electricity
The Peltier technology for converting Electricity to heat flow has been well established (Mahan 2016, Snyder 2008, Cengel 2016, Chen, 2008). Our group at CSET at Unisa have recently developed a prototype system that converts thermal energy heat flow directly to electricity using imported Peltier cells and is shown in Fig. 7. The unit contains basically a large aluminum pod system containing an air-cooled heat sink, an intermediate layer containing a series -parallel combination of Peltier cells. A “pod” of solid aluminum is connected by direct contact to a heat source, such as, a metal plate heated by solar irradiation, or heated by thermally heated water. We developed about a 25% increase in conversion efficiency as compared to other published works through developing a special electronic charge extraction system (results as demonstrated in Fig. 7(b)). ( RSA Patent 03803, 2019). Figure 6 (b) shows power transfer plots for one thermal to electricity (TE) cell to a fixed resistive load of 20 Ω and a delta T of 80°C across the sides of the cell. And for a load resistance from 0.5–4 Ω .
If a portion of the collected thermal energy could be converted to electricity, most of the electrical appliances in the household, especially the lighter loads, could be served with this energy. Smaller urban and rural houses obviously use less electrical energy, and they could even operate these systems on a completely self-sustainable basis.
The Thermal-to-Electricity Converter System (TECS) technology as developed in this study allows for easy upscaling by incorporating more TE cells into the converter. TE cells of the type as investigated in this study can currently be bought on the Web from overseas suppliers for USD 3.5 (roughly ZAR 50) per cell (Thermonamic Electronics, 2019, Communica Pty Ltd, Johannesburg, 2019). If the system is expanded to use 100 cells, coupled to a conventional domestic geyser hot water reservoir system, continuous power of 150 W (about 3 kWh per day) could be generated. A capital outlay of R10 000 and an estimated lifetime of the product over 20 years, will amount to ZAR 0.80 per kWh, when used in combination with other energy supply systems in SA Household context (Snyman et al, 2022). The lifetime of the TE systems developed in this study is estimated at 20 years, since no movable or seriously degrading materials are used.
The estimated savings associated with the utilization of thermal energy to electricity conversion in South Africa has recently been published for various ratios of replacing Eskom grid electricity supply (Snyman and Maeko, 2020). Analysis shows that, on average, a household consuming about 1 000 kWh per month, and augmenting grid electricity supply with a 75% TE conversion system, can fully recover its capital outlay within 1.5 years. After this, the household could have a continuous saving of about 80% of its monthly electricity bill if the unit is combined with a conventional solar thermal energy absorber system for heating hot water
Furthermore, most attractive feature of this technology is, the ability to store electricity for extended periods of time in standard water reservoirs Water has a very high heat capacity to store thermal energy, and pur analyses show that it is indeed a very low cost wai too store energy, as compared to most to much more complex “battery” technologies as are available on the South African market.
Another attractive feature of the system, is to convert thermal energy to electricity on a 24 hour basis per day, as the heat in the reservoir is stored quite efficiently with only about 10 loss of energy during a typical night in South Africa of 15 degrees Cecius ambient temperature. This feature also offers a consistent supply of initial solar energy to the household over several days, even in case of bad weather. .
6.3 Stored Liquid Petroleum Gas systems
Liquid Pertroleum Gas (LPG) (Alvi et al, 2016) is prepared by refining petroleum or "wet" natural gas, and is almost entirely derived from fossil fuel sources, being manufactured during the refining of crude oil, or extracted from petroleum or natural gas streams as they emerge from the ground. It currently provides about 3% of all energy consumed in South Africa, and burns relatively cleanly with no soot and few sulfur emissions. As it is a gas, it does not pose ground or water pollution hazards, but it can cause air pollution. LPG has a typical specific calorific value of 46.1 MJ/kg compared with 42.5 MJ/kg for fuel oil and 43.5 MJ/kg for premium grade petrol (gasoline). However, its energy density per volume unit of 26 MJ/L is lower than either that of petrol or fuel oil, as its relative density is lower (about 0.5–0.58 kg/L, compared to 0.71–0.77 kg/L for gasoline).
An advantage of LPG is that it can be stored with realative good saftey outside of households, and transported through thin piping into households and also between different households.
The cost of LPG is also very competative, 40 ZAR per kg containing 46 MJ of energy. This amounts to R3.15 per kWh (as derived from Table 2 as above). This makes it quite competative energy source to be utilised together with grid electricity and PV energy supply for application in SA households. However, natural gas or gas manufactured from green pastures, or suger cane plantations in South Africa, or socalled “green hydrogen” energy supply systems is quite viable to replace LPG in existing infrastructure systems in South Africa. .
6.4 Wind Turbine Energy Systems
UNISA, School of Engineering did pioneering reserch by reseaching the average wind speed in specific environments and then design small blade design sytems and turbines in order to extrcat the optimum amount of energy from the vailable wind energy. Such turbines have been designed for a medium scaled house in the Soweto township in South Africa, and now supplies energy to the household at alamost an sustainable basis. Prototype 2, with seven blades with a pitch angle of angle of 10° produced the maximum output power of 12,5W at 4.2km/h in late afternoons. The cost-efficiency of the system was determined as R1.80 per kWh (Snyman and Sithole, 2022).
Our analyses in Table 1 shows that the medium-sized urban household in South Africa uses approximately 500 kWh for basic electricity per month, or 10–15 kWh per day for household purposes, excluding water-heating. Water heating consumes about 10–15 kWh per day, or about 400–600 kWh per month.
The cost of thermally harvested and stored energy amounts to about R0.03 per kWh as outlined above in Section 6.1 It hence makes sense to replace all equipment and uses in the household that uses hot water (showering, bathing, clothes washing, dishwashing, diverse other utility uses), with thermally harvested energy, instead of using expensive and more expensive technologies to heat water. Liquid petroleum gas, compressed gas, hydrogen and heat pumps, are all, according to our analyses as above, cheaper energy forms that can be used to replace normally electrically driven heavy loads in the SA household.
Table 5, hence, presents a exemplary longer-term strategic design of household energy supply systems, that uses a more hybrid approach in supplying energy systems to a SA household. Derived energy costs as derived in the various above sections are uses to calculate the final estimations. It can be seen that by using a hybrid supply approach and carefully selecting energy sources, and by also giving preference to “green energies” that current costs of using “grid and national electricity” only can be about halved (approximately 50% of grid electricity price).
Also, one should consider the predicted energy cost raisings for grid electricity, cost reduction in PV systems and other renewables as realised in the last five years and incorporate these in order to is such longer term analyses (e-Tshwane, 2023; Eskom Residential Appliance Calculator, 2023, Eskom Tariffs and Charges Booklet 2023). Table 6 do such a prediction analyses. The data and projections as contained in these analyses could be most helpful for consideration for the strategic conversion of energy supply systems in existing households as well as in the design of new households in South Africa. An conservative rise in grid electricity of only 5% per annum over twenty years have been assumed, although the published statistics as in Fig. 1, predicts much steeper rises. Also, a cost inflation rate of only 2% per annum over twenty years have been assumed, considering that market pull forces may offset the costs of such systems as compared to the normal expected inflation rate of about 6% in SA over the next number of years. This analyses, also show that following a hybrid design and implementation policy for the future seems to provide the best longer term cost efficiencies.
Table 5
Costs for a hybrid energy supply system that can be applied to a typical SA medium scale household using hybrid green energy supply technologies in South Africa.
Energy Load Type
|
Estimated Whrs per week needed
|
Suggested Energy supply system
|
Estimated Total Cost per Month
|
Light Loads:
|
|
|
|
Lighting
|
20 lights @ 10 Watt per light @ 5 hours per day
|
TECS (30 kWhs @ R0.80 per kWh)
|
20
|
Television (TV)
|
300 Watt @ 4hours per day
|
PV (40 kWhs @ R2.80 per kWh)
|
110
|
Information and Telecomunication
(ICT)
|
50 Watt @ 5 units @ 15 hours per day
|
TECS (4 kWhs @ R1.50 per kWhs)
|
10
|
|
Subtotal
|
60
|
R140
|
Medium Loads:
|
|
|
|
Microwave oven
|
1000Watts @ 3 hours per day
|
PV (100 kWhs @ R2.80 per kWh)
|
280
|
Fridge-Freezer
|
350 Watts @ 15 hours per day
|
PV (150 kWhs R2.80 per kWh)
|
420
|
Microwave Oven
|
2000 Watts @ 2 hours per day
|
PV (100 kWhs @) R2.80 per kWh)
|
280
|
Electric Fencing
|
50 Watts @ 24 hours per day
|
PV (40 kWhs @) R2.80 per kWh)
|
110
|
|
Subtotal
|
400
|
R1100
|
Heavy Loads:
|
|
|
|
Hot Water Geyser
|
2200 Watts @ 5 hours per day
|
Solar Absorber (300 kWhs) @ R 0.50 per kWh
|
150
|
Heater Element Oven and plate pods
|
2200 Watts @ 1 hours per day
|
Gas (60 kWhs @ R3.15 per kWh )
|
200
|
Kettle
|
2200 Watts @ 0.5 hours per day
|
Small Heat Pump (30 kWhs @ R1.50 per kWh )
|
50
|
Swimming Pool Pump
|
750 Watts @ 6 hours per day
|
Compressed Gas (200 kWhs @ R0.50 per kWh)
|
100
|
|
Subtotal
|
600
|
R500
|
|
Total
|
1050
|
R1700
|
Table 6
Projected costs per month for various energy supply systems as a function of estimated economic inflation per annum in South Africa.
Energy System Utilsed
|
Initial
Outlay /
Cost per month
|
Estimated Inflation per annum
%
|
Cost per Month
5yrs
|
Cost per Month
10 yrs
|
Cost per Month
20 yrs
|
All grid
|
1000kWh/R3000
|
+ 5
|
3828
|
4886
|
7959
|
PV Only
|
1000kWh/R2300
|
+ 2
|
2539
|
2800
|
3417
|
50%
Hybrid
|
1000kWh/R1700
|
0
|
1700
|
1700
|
1700
|
100% Hybrid
|
1000kWh/R1000
|
0
|
1000
|
1000
|
1000
|