Average monthly irradiance.
The following energy sources, in a various combinations were assessed to provide potable water using seawater reverse osmosis (SWRO) for around 50,000 people in Eritrea: wind power and solar power. Various types of SWRO technology were employed and the cost of scenarios that were able to meet the users’ water needs was compared with the costs of the equivalent diesel generator powered scenario over 25 years. The most financially-attractive scenario, a hybridised power plant using solar and wind power was compared with the equivalent conventionally (diesel generator) powered scenario using present and net present value (NPV) methodology. The discount rate used for NPV calculations was found to be pivotal for this comparison, so the logic of the appropriate discount rate was investigated and a discount rate of 3.6% was considered the most appropriate. This resulted in the renewable powered solution for this scenario being financially attractive when compared to the diesel generator powered scenario. This conclusion was mainly due to recent changes in the prices of diesel fuel in Eritrea and solar power generally. Research conducted on this scenario previously, and published in 2014 based on 2010 prices, concluded that this scenario was not financially attractive in comparison to diesel power.
- reverse osmosis desalination
- solar power
- wind power
- renewable energy economics
- discount rate
Renewable energy is currently considered by many to only have viability for a small portion of energy delivery within a larger system due to intermittency.
This intermittency potentially undermines the environmental and energy security advantages on offer from renewable energy and decentralisation of supply.
Sets out the investigation of the use of renewable energy sources in such a way that they could be justified for use without reliance on conventional energy sources and to stand alone as an independent and viable power source in their own right; and
Is a continuation of research initially conducted in 2011 and subsequently published in 2014 , to reflect the impact of changes to diesel fuel and solar photovoltaic prices since 2011.
The scenario used to investigate the technical and financial viability of renewable energy was its use to power reverse osmosis (RO) desalination plants to provide water for the personal use of 50,000 people.
1.1. The location
Eritrea was selected due to its susceptibility to droughts, and consequential loss of life. The hypothetical pretext for the need for municipal desalination is that Eritrea has a substantial coastline, and the sea level rise expected due to climate change has the potential to hasten the intrusion of saline water into the fresh groundwater aquifers in the coastal zone. The focus of this research will be the island of Massawa, shown below, which is in a particularly dry part of Eritrea with, typically, less than 200 mm of annual rainfall as shown in Figure 1. This is in comparison to the UK, where according to the Metrological Office , the minimum rainfall between 1981 and 2010 is around 600 mm.
2. The modelling exercise
The modelling exercise was conducted in 3 main stages as shown in Figure 2.
2.1. Stage 1
Stage 1 employed Solar PV with the No Brine Stream Recovery (BSR) RO plant only as shown in Figure 3.
The methodology used to identify the minimum number of membranes that the RO plant would require is shown in Figure 4.
The initial results (shown in Figure 5) gave the minimum number of membranes required to produce 7000 m3/day, if the plant were run for 24 h continuously at each recovery ratio.
It was decided that the RO plant would operate where the minimum number of membranes required was consistent between 15 and 24% recovery ratios, as shown in Figure 5.
A schematic diagram of the No BSR plant employed for the modelling within this research is shown in Figure 6.
The resulting No BSR RO plant operating profile, including impacts on efficiency as load changes as expected due to intermittent power as indicated in the US DOE Tip sheet 2 , is shown in Figure 7.
This surface was mapped mathematically using polynomial equations and the method used to calculate the amount of water produced, was a ‘for’ loop in Matlab, as shown below:
where Pg 1 = the power available to operate the RO plant at each hour during the year; index(i) identifies the location of the prevailing seawater temperature for each hour of the year; ppolycoeff is a file that contains all the polynomial equations relating to each 0.01°C step from 3 to 42 °C; i=1:rwr defines the number of times that the calculation should be conducted before stopping; i=the number of the calculation being conducted, in this case, conducted in sequence from 1 – (rwr) the max number of which is 8760 (the number of hours in a year); Polyval is the matlab function that then evaluates the polynomial equation identified by (index(i)) making the corresponding Pg at (i) the subject.
2.2. Solar power
Massawa is one of the hottest inhabited places in the world, so solar PV was adopted.
|Month||Original monthly average (W/m2/day during that day)||Conversion to kW/h||Clearness index* applied by HOMER|
2.2.1. Renewable energy system employed within model
The solar array for this process assumes that 10% of the available radiation, at any time when the sun is shining, is captured and converted to usable DC electrical power. This DC power is then be converted, by an inverter, to AC power, suitable for use by the RO plant. The efficiency of this power conversion was taken as between 90 and 95%.
Sufficient solar photovoltaic power was installed so that the maximum power output during the year from the solar PV would achieve the maximum flowrate of the RO plant. Additional power was then added in discrete levels, up to (and including), the power required to achieve five times maximum flowrate of the RO plant.
2.3. Stage 2
Stage 2 employed the same methodology as Stage 1 (application of solar PV only), but for the BSR RO plants (Pelton wheel and Pressure exchanger).
2.3.1. Pelton wheel
The Pelton wheel RO plant system modelled is shown in Figure 8.
As shown in Figure 8, the Pelton wheel BSR RO plant design utilises the brine/concentrate stream to power a Pelton wheel turbine, which is mechanically linked to a high pressure pump arrangement and partially pressurises the incoming feedwater. This reduces the external power required to raise the feedwater pressure. The resulting Pelton wheel BSR RO plant water production profile, at varying input power and feedwater temperatures, is shown in Figure 9.
2.3.2. Pressure exchanger
The pressure exchanger RO plant system modelled is shown in Figure 10.
As shown in Figure 10, the pressure exchanger BSR RO plant uses the brine/concentrate stream to pressurise a hydraulic chamber. This hydraulic chamber acts on a piston arrangement, which in turn is used to partially pressurise the incoming feedwater. A booster pump then raises the now partially pressurised feedwater to the correct pressure to combine with the feedwater pressurised by the high pressure pump for desalination by the RO plant membranes.
The resulting pressure exchanger BSR RO plant water production profile, at varying input power and feedwater temperatures, is shown in Figure 11.
As was the case in Stage 1, additional solar PV power was added in discrete levels up to (and including) the power required to achieve five times maximum flowrate of each of the RO plants.
2.4. Stage 3
Stage 3 modelled the addition of wind power in an attempt to make the renewable powered scenario competent to produce the correct amount of potable water for the Massawa residents.
2.4.1. Wind resource available Massawa
The monthly average wind speed data at Massawa was taken from monthly average data over 4 years based on local weather reports  and applied to HOMER to derive the wind speed for each hour of the year, shown in Figure 12.
As was the case in the previous Stages, additional wind power was added in discrete levels up to (and including) the power required to achieve five times maximum flowrate of each of the RO plants.
2.5. Scenarios modelled and scaling of renewable energy scenarios
Sixty scenarios were modelled with BSR and No BSR RO plants limited to 7000 m3/day output capacity with varying installed solar PV and wind capacity.
There was limited success in identifying scenarios that were able to meet the water demands of the local population without increasing the capacity of the RO plant to compensate for intermittency.
So, each of the RO and Power plant scenarios were scaled up by the ratio of water shortfall, i.e. if the combined RO plant and Power scenario made 50% of required water, both the RO plant and installed power are doubled in size to fully meet the water demand requirements.
Prices are based on exchange rate of $1–£0.636 as was the case in 2011, when much of the original research was conducted.
3.1. RO plant and reservoir costs
Reservoir costing was taken as £82,115,200 based on extrapolation of various reservoir costs from  to meet the required holding capacity.
3.1.1. Impacts of intermittency
Intermittent operation of desalination plants is possible and has already been realised in smaller systems as shown in [12, 13]. According to Rizzuti , however, it is understood that for large-scale seawater desalination plants, the plant’s lifetime could be reduced by increased scaling, fouling and corrosion.
This said, there is potential for the mechanical wear aspects on the plant to be reduced due to the increased size of the scaled up renewable RO plant, as the components will not experience the stress of operating at full load for as much of the time. It was considered reasonable to conclude that the impact on RO plant operation and costs will vary based on the power supply intermittency. The degree of this impact is not well understood and is worthy of further investigation, but based on the research conducted, it is considered appropriate to increase O&M costs to acknowledge the impact of intermittency. Therefore, in the light of the lack of information to support accurate estimates, a pessimistic assumption was employed for intermittent operation that annual O&M costs increase by 100% over those of a continuously operated plant on a per m3 of water produced basis increasing the costs as shown in Table 3.
|(£x106)||No BSR||Pelton wheel||Pressure exchanger|
|Total costs over 25 years||48.8||79.1||56.0|
|Operating cost for||No BSR (£)||Pelton wheel BSR (£)||Pressure exchanger BSR (£)|
|Continuous operation (£/m3)||0.62||1.08||0.70|
|Intermittent operation (£/m3)||1.24||2.16||1.40|
|Intermittent operation/ year (£x106)||3.16||5.52||3.58|
|Intermittent operation over project life (£x106)||79||138||90|
Table 4 shows the CAPEX and OPEX costs associated with the renewable energy sources employed including the reduction in the price of solar PV between 2010 and 2016.
3.2.1. Reduction in solar PV price in 2016
The costs employed for 2017 were based on the ‘utility scale PV, fixed tilt (100 MW) as shown on the right hand side of Figure 13. $1.42/W = £903/kW.
This was considered to be an optimistic price due to Massawa’s relatively remote location and limited market potential and so the supply chain cannot be relied upon to be developed to a stage where it provides competitive pricing. The NREL webpage  shows deviation of +/− $694/kW about a mean cost of $2025/kW, which would make the worst case cost $2719/kW (£1729/kW).
So for the purpose of this report the CAPEX for the plant was taken as £1700/kW, which assumes that the PV costs are at the higher end of the range.
3.2.2. Operations and maintenance costs
The NREL webpage  indicates the O&M costs for a PV system between 1 and 10 MW are $16 +/−$9/kW, which equates to a high end cost of $25/kW (£15.90/kW).
The Operation and Maintenance (O&M) costs for the plant were taken as £16/kW, which assumes that these costs are, once again, at the higher end of the range, due to the expected lack of a competitive local market.
3.3. Conventional power costs
Table 5 shows the CAPEX and OPEX costs associated with the diesel generator power plant modelled at Massawa, which is based on various sources.
3.3.1. Increase in diesel fuel prices in Massawa
|Diesel generator costs for 25 years based on 2010 fuel price||Diesel generator costs for 25 years based on 2016 fuel price|
|Installed power costs (£) ||Fixed O&M costs (£) ||2010 fuel costs (£×106) based on £0.71/l ||Total scenario costs (including RO plant and reservoir) (£×106)||2016 fuel costs (£×106) based on £1.75/l||Total scenario costs (including RO plant and reservoir) (£×106)|
|Date||Price ($)||Price (£)|
As can be seen from Table 6 the price of diesel fuel in Eritrea has increased 10 fold between 1992 and 2014, the latest date that price information is available from the sources used. 2014 was a relatively high price point when in the UK diesel fuel was around £1.29/l, but by June 2017 the price of diesel fuel in the UK had fallen to £1.18/l, a reduction of 8.5% due, in part, to the fall in crude oil prices. Although the influences in the price of retail diesel fuel are affected by different variables in the UK and Eritrean economies, for the purposes of this research a reduction of 8.5% of the 2014 price has been adopted for the price of diesel in Eritrea to account of the fall in crude oil price. This results in an estimated price for 2017 of $2.75/l (1.75/l) as shown in Table 6.
Shown in Table 7 are the results for the costs of the most financially attractive scenarios for each stage of modelling power source and RO plant type in comparison with the diesel generator powered plant.
Scenarios that have become financially viable (are cheaper than the diesel generator powered equivalent), due to the application of latest diesel fuel and solar PV prices are highlighted in yellow and the most financially attractive is in ‘bold’ font.
The changes in solar PV and diesel fuel prices have made the renewable powered scenario much more financially attractive than in 2010, but only the renewable powered No-BSR RO plant scenarios have actually become financially attractive in comparison to the diesel generator powered scenario (Table 7). The most financially attractive scenario is estimated to cost 77% (around ¾) of the cost of the conventionally powered plant over the project’s 25-year life.
The net present value (NPV) is a central tool in discounted cash flow (DCF) analysis, where each cash inflow/outflow is discounted back to its present value (PV). Then they are added together. So, NPV is the sum of all the terms:
where t - the time of the cash flow; i – the discount rate (the return that could be earned on an investment in the financial markets with similar risk.); the opportunity cost of capital; Rt - the net cash flow (the amount of cash, inflow (value of water sold) minus outflow (the cost to maintain the power source and RO plant) at time t.
The next two sections will derive the following to allow the NPV to be calculated.
Price of water
5.1. Price of water
As part of an informal telephone conversation, Tesfai  stated that municipal water in Eritrea costs less than 5p/l, and bottled drinking water costs less than 10p/l.
The Munich Re Foundation  also states that:
‘Water from tank trucks costs 15 Nakfa or about €0.90 per 20-litre canister’.
This equates to 3.8p/l or £38/m3, which is in keeping with Tesfai’s estimate of less than 5p/l or £50/m3.
The situation described above is borne out by Awate , which reports rationed water being delivered by bowser in Asmara, the capital of Eritrea in 2017, and the quality of the water provided being saline/brackish and requiring disinfection before consumption, which in turn, due to incorrect disinfection dosing, could have longer term health effects.
For the purposes of this research, the cost of water to the end user at Massawa will be taken as £38/m3 based on the estimate from the Munich Re Foundation , but as the RO plant will, in effect, only be a different water supply point for the tankers to collect their water from, cannot be given credit for the full £38/m3 that the end user pays.
The overall cost for the most financially viable renewable powered scenario in 2010 based on comparison to the equivalent diesel powered scenario is around £325 million over 25 years, and to break even each cubic metre of water delivered by the RO plant would need to be priced at £5.05, which incidentally is more than 3 times the cost of water in the UK .
For the purposes of this research, the cost of the water delivered at Massawa will be £5.05/m3.
5.2. Discount rate
The normal method for calculating the discount rate for UK government financed infrastructure projects, according to the UK Treasury , is 3.5% above the inflation rate.
The inflation rate has varied considerably in the UK over the last 10 years as indicated by the graph on the ‘Inflation EU’ Website  which shows an average high of 4.8% in 2011 and a low of 0.05% in 2015. This gives an average of 2.375% average inflation over the last 10 years. The inflation rate adopted for this calculation is 3.5% to include a margin for the inflation risk over the 25-year term of this project. This figure is in keeping with the long-term rate described in the UK Treasury’s ‘Green Book’  on p. 26.
For the purposes of this research, the discount rate was taken as the sum of these two parts (7%).
5.3. NPV results
Shown in Figure 14 are the accumulated costs for the most financially attractive renewable powered scenario at Massawa (Solar plus Wind power), and the equivalent No BSR RO plant Diesel Generator-powered scenario.
Having set the price of the water revenue at the cost for the break even of the renewable powered scenario, it is shown to break even at the 25 year point, and the diesel generator powered scenario makes a profit of over £50 million over the life of the project.
When the scenario is updated to reflect diesel fuel and solar PV prices in 2016/7 the scenario changes significantly as shown in Figure 15.
As can be seen from Figure 15, the updating of diesel and solar PV prices alters every aspect of the financial viability of the most financially attractive renewable powered scenario, in that:
It now breaks into profitability;
It is significantly more financially viable than the diesel generator scenario; and
The end of life profit for the diesel generator scenario of more than £53 million has now turned into a loss of almost £70 million.
Figure 16 shows the difference that applying the 7% discount rate makes to the financial attractiveness of the scenarios.
As can be seen from Figure 16, the application of the 7% discount rate means that neither scenario is now profitable, and further that the renewable powered scenario (PV + Wind) is now more than £28 million less financially attractive than the diesel generator powered scenario.
In selecting the appropriate discount rate for long-term public policy decisions, economic theory tends to distinguish between two components.
The rate of pure time preference is the discount rate that would apply if all present and future generations had equal resources and opportunities.
In addition, there is a wealth-based component of the discount rate, reflecting the assumption that if future generations will be richer than we are, then there is less need for us to invest today in order to minimise the financial burden on those that follow.
In the notation of ‘The Stern Review’ , the discount rate, r, is the sum of these two parts:
where δ (delta) is the rate of pure time preference; g is the growth rate of per capita consumption. If per capita consumption is constant, implying that g = 0, then the discount rate r = δ; η (eta), determines how strongly economic growth affects the discount rate. A larger value of η implies a larger discount rate, and hence less need to provide today for future generations (as long as per capita consumption is growing).
Stern takes the position that all future generations should be treated equally, except that there is a small probability that future generations will not exist – for example, if a natural or man-made disaster destroys most of, or the entire human race. The probability of destruction of humanity is taken by Stern as 0.1% per year; pure time preference (δ) is therefore set equal to 0.1%. That is, we are only 99.9% sure that humanity will still be here next year, so we should consider the well-being of people next year to be, on average, 99.9% as important as people today. Stated simply, the only reason that the current generation should not consider the needs of those in the future is due to the small possibility that the future generation will not exist, not because the future generation will be rich enough to manage the previous generation’s impact on the environment.
To calculate the discount rate, Stern estimates that the growth of per capita income will average 1.3% per year, and sets η = 1 to indicate that future generations are not richer than the current generation. Therefore, the Stern Report discount rate is:
This makes a marked difference to the viability of the renewable powered scenario, as shown in Figure 17.
As can be seen from Figure 17, the reduction of the discount rate means that, although neither scenario is financially viable, the renewable powered scenario is now more than £45 million more financially attractive than the diesel generator powered scenario.
5.4. An alternative point of view
Taking the Stern position that this generation can only apply 0.1% to the discount rate, it does not seem entirely unreasonable to suggest a discount rate of 3.6% based on:
Long-term view of inflation (3.5% as derived previously) + 0.1% based on probability that next generation will not be here
This would give appropriate recognition to the fact that this project represents a significant CAPEX investment to:
Improve the quality of the lives of users, and in so doing;
Minimises the impacts of climate change and the associated potential costs for future generations, whilst;
Acknowledging a realistic view of inflation over the longer term of project’s of this type for the current generation.
Figure 18 shows the comparison of the renewable and diesel generator powered scenarios.
As can be seen from Figure 18, the 3.6% discount rate does not make either option financially viable, but does make the renewable powered scenario around £9.2Million more financially attractive than the diesel powered scenario.
Overall conclusions are that:
It is technically possible to desalinate water for human consumption at Massawa using renewable energy.
This is financially viable in comparison to the use of diesel generators, but only recently due to the changes in solar PV and diesel fuel prices; but
The NPV methodology for evaluation of renewable energy projects has a significant impact on the financial viability of such projects; and
The operational impacts of using intermittent power sources on municipal RO plants needs to be better understood.