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Pump as turbine – A pico-hydro alternative in Lao People's Democratic Republic
Renewable Energy 35 (2010) 1109–1115
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Technical Note
Pump as turbine – A pico-hydro alternative in Lao People’s Democratic Republic Mariano Arriaga* Instituto de Investigaciones Ele´ctricas, Gerencia de Energı´as No Convencionales (Electrical Research Institute, Non-conventional Energy Division), Calle Reforma No. 113, Col. Palmira, 62490, Cuernavaca, Morelos, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 March 2009 Accepted 25 August 2009 Available online 27 November 2009
This paper presents the pico-hydro development status in Lao PDR and introduces the Pump as Turbine (PAT) concept as an alternative for isolated communities (40–500 people). The intention is to provide a long-term reproducible system for communities where pico-hydro propeller turbines are insufficient and proper turbines are expensive. This approach presents a high quality and cost-effective solution for rural electrification which can be installed, commissioned, and maintained by local staff and villagers. Furthermore, a 2 kWel PAT-scheme is proposed for a community in the Xiagnabouli province and considers power generation alternatives, sizing, asynchronous motor simulation, civil works, cost estimation, and social aspects. Ó 2009 Published by Elsevier Ltd.
Keywords: Asynchronous machine simulation Lao PDR Pico-hydro Pump as turbine Renewable energy rural electrification
1. Introduction The Lao People’s Democratic Republic is a land-locked country located in South East Asia. In 2007, Laos had a population of 5.8 million people with a Gross Net Income per Capita of 580USD; which the World Bank classifies as a low income country. As a result, the country has heavily relied over the years on foreign aid for its development. In 2005, the Lao National Statistics Centre estimated that 50% of the households had access to the electrical national grid, 10% had electrical power through their own generator or car battery, and 40% had no access to any electrical source. The electrification level for cities and other densely populated areas is 90%, whereas small isolated communities have a considerably lower level. Thus, only 43% of the rural population with road access has electricity and 11% where no-road access exists [1]. The latter category is not likely to increase its electricity access in the coming years; since, as discussed in Ref. [2] rural poverty is remote and heightened when there is no road access. These percentages need to be taken carefully due to two main reasons. Firstly, the lack of specification for pico-hydro, solar home systems (SHS), diesel engines, or any other source of electricity. To date there is no official/reliable count for such systems in the country. Secondly, the statistics do not consider electrical supply reliability or quality level. In a surveyed community, the voltage variation is between 140 and 250 V and the * Cerro de San Andres 239, Campestre Churubusco, Coyoacan, Mexico City 04200, Mexico. Tel.:þ52 55 5549 5356, þ777 362 3811x7253. E-mail addresses: [email protected], [email protected] 0960-1481/$ – see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.renene.2009.08.022
frequency variation between 38 and 55 Hz due to the lack of maintenance and/or seasonal rainfall variation. In Laos, 97.5% of the electricity generation comes from hydro power and the country is currently increasing its capacity, mostly driven by electrical exports to Thailand and China. The total untapped planned capacity for 2006–2011 is 4013 MW and 2226 MW from 2011 to 2020 [3]. These hydro plans are dealt with at the macro-economic level and do not consider off-grid/small communities where electrical generation and/or grid extension costs deters any feasible commercial approaches.
1.1. Pico-hydro technology Pico-hydro schemes (<5 kW) have been successfully implemented in remote regions in the world [4–8]. Their common objective is to provide a cost effective and simplified alternative to supply electricity to areas that are relatively far from the electrical grid. The types of turbines and generators vary depending on the local conditions, budget, and equipment availability. For high water head (HN > 30 m) areas, a common approach is to locally manufacture Pelton turbines [7,9]. However, Laos’ topography has limited areas where the head meets this requirement. For low head sites (<2–3 m), run of the river schemes include the massproduced Pico-hydro propeller (PHP) turbine coupled to a synchronous permanent magnet generator [8]. These PHP units are found in markets and hardware stores and their capacity ranges from 0.2 kWel to 3 kWel and their price ranges from 35USD to 300USD, respectively. In addition, there are mini-hydro turbine manufacturers outside Laos
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which provide a high quality and efficient unit which can be easily installed and maintained. Their manufacturing costs are frequently higher than all the previously mentioned approaches since currently there are no local businesses in the country. 1.1.1. Pump as turbine (PAT) Thoma presents the first published work regarding pumps running in abnormal conditions [10]; such work is retaken by Thode to outline the use of standard pumps running in reverse for electrical power generation [11]. The paper states the advantages of the PAT and remarks that radial, mixed and axial centrifugal pumps can be successfully operated in reverse when a design review of the pump characteristics is done to ensure proper operation under s¸oiu presents a brief description turbine operating conditions. Isba of PAT operation and related equations; however, no experimental data is provided to validate the model [12]. Derakhshan presents a thorough theoretical model for mixed flow PATs, in which several dimensioning parameters of the pump are required to calculate the expected efficiency [13]. The results presented show a precise model to characterize low specific speed (Ns) turbines 15–55 rpm [m, m3/s]. Similarly, Gu¨lich presents a complementary framework for units ranging from 10 to 200 rpm [m, m3/s] and the turbine/ pump operation based on the unit’s specific speed (Ns), flow rate (Q), head (H), and Torque (M) [14]. In Section 3.3, these theoretical frameworks are applied to calculate the expected operational point and efficiency of the selected PAT. Additionally, Rawal presents a Computational Fluid Dynamics (CFD) simulation for a mixed flow PAT (Ns ¼ 94 rpm [m, m3/s]) However, at this stage the results cannot be generalized and still require further modeling [15]. From a practical perspective, Williams presents a remarkable guide for sizing PAT units, the empirical equations to calculate the expected output, and a brief troubleshooting guide [16]. The guide also mentions the alternative of reducing the impeller’s diameter to reach a closer to optimal operating point for the pump running in reverse. Finally, Maher and Greacen present successful PAT projects used for rural electrification in developing countries [4,5]. 1.2. Off-grid power in the Lao PDR Smits makes a conservative estimation of the number of PHP turbines within Laos, based on empirical data [17]. It is estimated that 60,000 units are installed, mostly in the Northern provinces, providing electricity to 90,000 people. Though, there are commonly identified failure causes including core demagnetization, burn out windings, and worn upper/lower bearings. This latter failure mode is the most widely found and, in particular, the lower bearing. The PHP units are a respectable available option when connecting one household these units are rarely used to connect numerous homes. Recently, a hybrid system was installed in the Xiangkhoung province which consists of a 12.5 kWel mini-hydro unit, 1.5 kWp PV panels, and a 10 kWel diesel genset [18]. From a technical perspective, the system successfully supplies a 100 household village with a constant and reliable source of electricity. However, the social structure and electrical consumption forecast have not developed as expected. The system has an average use of 10 kWh/ day and it is estimated that there is only a 4–5 kWel peak power which results in a power utilization of 20% during the peak hours. Thus, from an economical perspective the project cannot be replicated in similar conditions. In summary, remote rural electrification in Laos requires approaches that cover several technical issues, such as cost effectiveness, repeatability, easy installation and maintenance, local availability, and reliability; which, for certain sites, can be accomplished by a PAT scope. Furthermore, from a technical and economical view, this alternative bridges the gap between the PHP
and cross flow turbines. In chapter II, this paper estimates the potential in Laos for PAT installations. Chapter III presents the natural resource assessment, system design, PAT selection, energy source, energy consumption profile, and induction motor simulation. Chapter IV outlines the estimated cost for the civil, electrical, and mechanical works required and chapter V gives a general overview of the social implications and long-term maintenance. 2. Pico-hydro villages 2.1. Target village estimation for PAT units The following calculations use the electrical consumption data acquired from the hybrid system: E ¼ 0.19 kWh/(day$household) and Ppeak ¼ 75 Wel/house. The PAT electrical power range considered is 1–12 kWel at the generation point. The lower limit is set by the available PHP turbines and the upper limit is set by the existing power controller capacity. Assuming a design factor of 0.85, the size of the village that can be electrified ranges from 10 to 140 households (40–500 people) solely using a PAT system. The main issue involves covering the electrical power peak and not the energy demand which can be overcome with certain design modifications. The first alternative includes a mini-grid with a central battery system. The second alternative is to use a battery charging station [6] which has economic advantages. Currently, there is an ongoing effort to implement a solar battery-lantern project in Laos [19] which at this stage, aims to define the consumer patterns and technical specifications for the system. Next, the design, simulation, and dimensioning for a specific PAT project is presented including the discussed design alternatives. 2.2. Specific site selection Doikao is a small village with 86 households located in the Xiagnabouli province (19 230 N 101710 E). The village is considered poor with an average household income of 180USD/year (0.5USD/ day) which seldom covers the basic needs – for reference a liter of potable water costs 0.2USD. The current sources of lighting are candles 4.7USD/month and kerosene lamps which, on average, consume 1.5 l a month and costs 2.24USD/month. However, there is a significant disparity between this cost and what they are willing to pay for electricity: two lights 0.23–0.59USD/month and four lights/CD player 1.17USD/month [20]. 3. Renewable energy system sizing 3.1. Natural resources in Doikao The potential natural resources for power generation are solar, wind, and hydro power. The solar irradiation is approximately 4.9 kWh/(m2$day). As for the wind resource, the northern areas have poor wind velocity (<5.5 m/s) which cannot be utilized as a significant source of power generation. As for the hydro resource, the village has a stream located 500 m away from the NGO office. A preliminary survey by Smits during the dry season measured a flow rate of Q ¼ 35 l/s and a gross head HG ¼ 12 m [20]. A single measurement for the river flow rate is not ideal; however, detailed information is complex to obtain since the site is not easily accessible. Still, using the dry season flow rate for a PAT design assures that the system’s output can be guaranteed for the full year. 3.2. Design phase The objective is to supply electrical power for lighting of the Doikao village and an NGO office located in the village. The most
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viable alternatives for electrical power generation are solar and/or hydro power. The office is intended to be electrified directly by the pico-hydro system which loads include lights, laptops, refrigerators and communication equipment. In addition, the village can potentially be supplied by three alternatives: SHS, battery charging unit or a mini-grid system. 3.2.1. Option 1 – pico-hydro unit and SHS A SHS of approximately 20–30 Wp can supply each household with lighting. The advantage of this system is the simplicity of the ownership process and familiarity of the installation procedure in Laos. Nevertheless, this approach has a significantly low hydro system usage (51%); since approximately 22 kWh/day are directed to the dump load and 23.5 kWh/day are used by the actual loads. 3.2.2. Option 2 – pico hydro and battery charging The excess energy directed to the dump load can be used to run a battery charging station (600 Wel) to supply electric power to the village. For example, considering that the charging station runs for 10 h/day, one 12V/10Ah battery/household, and a system performance of 50%, the unit can potentially charge the equivalent of 25 households/day – 30% of the village. The main advantage is that no additional equipment is required. Each household would be responsible for its battery and a fee could be paid every time the battery is charged which in turn would generate income. From an energy point of view, this configuration results in a 61% hydro system usage, with 28 kWh/day directed to the loads and 17.5 kWh/ day directed to the dump load. 3.2.3. Option 3 – pico-hydro mini-grid system An electrical mini-grid scenario can supply power to the office and village using the pico hydro and a centralized battery bank. The office loads are kept unchanged and an average of 25 Wp/household is allocated. Furthermore, if an estimated battery power roundtrip efficiency of 73% is used, then 6.3 kWh/day are needed to cover the energy night peak which is lower than the 16.2 kWh/day energy surplus (dump load). Considering the 6.3 kWh/day, 1.9 days of autonomy, and 40% depth of discharge, a battery bank of 30 kWh (24V/1 250Ah) can be installed to cope with the demand peak. Fig. 1 presents the mean power profile for the three scenarios considered.
3.3. Pico-hydro turbine options 3.3.1. Pico-hydro propeller turbine The largest available turbine is 3.0 kWel which assuming 45% efficiency has an electrical output of 1.35 kWel at the generation point. Furthermore, the quality of the turbines is dubious and most of them present machined areas with visibly poor surface finish which, obviously, increases wear and decreases efficiency. 3.3.2. Cross flow turbine The main advantage is the flexibility of running the unit at different flow rates which, increases the power during the rainy season. Thus, there is currently no turbine manufacturer in Laos; which means that the construction and installation of the turbine is done by foreign staff. 3.3.3. Pump as turbine Besides the previously mentioned advantages, the system can be designed and installed by local staff, the pump distributors are locally available assuring spare parts on hand, and most importantly, the maintenance can be done by the villagers. However, the PAT approach has two main drawbacks: the fixed operational speed and the limited accuracy in the performance prediction. As a result, since the main objective is to provide a high quality, cost effective, long-term solution, the PAT alternative is selected as the most viable option.
3.4. Pump as turbine selection The design criteria for the hydro scheme is based on a flow rate of Q ¼ 0.035 m3/s and a gross head of HG ¼ 12 m; which is to generate electricity at 230/400 V–50 Hz. As for the available pumps, Laos has two major pump manufacturers: Kirloskar and Grundfos. The objective is to convert the pump curves and best efficiency points (bep) to its turbine equivalent. The following formulas are used to calculate the flow rate and head when running the pump on reverse mode [16].
Qt ¼
nt Qbep $ np h0:8 max
Ht ¼
Fig. 1. Daily energy profile.
1111
nt np
2
Hbep $
h1:2 max
(1)
(2)
where, Qbep and Hbep are the bep of the pump, hmax is the pump’s maximum efficiency, and np and nt are the rotational speed (rpm) when running as pump and turbine, respectively. These equations are based on the standard pump Affinity Laws. Since, these equations have an empirical nature; the actual PAT performance can potentially differ by 20% [16]. The system curves are calculated based on the commonly used Manning equation for head losses, the pump curves are taken from the manufacturers’ specifications and the PAT bep are calculated using (1) and (2) at maximum and 7% reduced efficiency and the design range is set at 0.035–0.040 m3/s. As a result, from the prospective PAT analyzed, PAT NB 80-160/175 and NB 100-250/236 have similar operational curves and are the closest to the design criteria (Fig. 2). Still, PAT NB 80-160/175 is selected for economic reasons. The characteristics of the pump are HP bep ¼ 8 m, QP bep ¼ 0.028 m3/s, hmax ¼ 80%, n ¼ 1445 rpm, p ¼ 4 poles, and slip ¼ 0.037. Based on these specifications in Eqs. (1) and (2), the corresponding parameters when running as turbine are HT bep ¼ 12 m, QT 3 bep ¼ 0.035 m /s, and n ¼ 1555 rpm, if the manufacturers efficiency is considered. However, if a more realistic efficiency is used
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current in the motor windings. Smith describes a methodology to calculate the required capacitance and its connection to obtain a stable one-phase operation by using a C–2C delta connection [22]. These procedure results in a self-excitation capacitance of C ¼ 23 mF and the expected stable operation point at P ¼ 1.95 kWel.
Fig. 2. PAT curves with bep at rated and decreased efficiency.
(h ¼ 73%), then the corresponding values are HT bep ¼ 13.4 m, QT m3/s. In addition, Derakhshan and Ramos, among others, have designed estimation methods based on the pump specific speed [13,21]. Each author uses a different definition for specific speed depending upon the parameters used for the calculation. The following are the respective equations used.
bep ¼ 0.038
NS ¼ Nm; m3 =s ¼
NP ¼ Nðm;kWÞ
np $
qffiffiffiffiffiffiffiffiffiffi Qbep
0:75 Hbep
pffiffiffi nP $ P ¼ 1:25 Hbep
(3)
(4)
3.5.1. Electrical system simulation The MAG model is built in Matlab/Simulink using the SimPower Systems Toolbox to represent a self-excited three phase squirrelcage asynchronous generator. The further motor parameters required are the stator and rotor resistance Rr ¼ 1.36 U, inductance Lls ¼ 5.8 mH, and inertia J ¼ 0.076 kg m2. The asynchronous machine power source is the mechanical torque, T ¼ 14.7 N m, provided by the PAT taking into account its mechanical efficiency, h ¼ 0.8, and rotor speed, n ¼ 1555 rpm, required when running as a generator. The excitation capacitance must be induced to create the magnetization current; in this case, a three phase capacitor bank model is used. Finally, a three phase maximum resistance of 2.4 kWel is connected as a load. This simulation represents a realistic model for an induction MAG; however, as any model, there are certain assumptions and limitations. The motor inductance values used are of a similar motor; since the manufacturer’s information is not available. In addition, the asynchronous machine model in Simulink is already ‘‘internally’’ wired as a wye connection which avoids the possibility of a C–2C arrangement to attain a one-phase stable system. However, this step is more important from a system-stability than from the excitation capacitance and induction load perspectives. The simulation time is intended to cover the start-up effect of the electrical system, t ¼ 1.2 s. The results from the simulation show that as the excitation capacitance increases, the frequency proportionally decreases and a similar effect happens with the efficiency (Fig. 3). Thus, during implementation, it is advisable to avoid over-compensation since this decreases the frequency, motor temperature, efficiency, and voltage. The previous results use a solely resistive load; then again, the system is intended to run both, resistive and inductive loads. The procedure followed to analyze the behavior of inductive loads is to maintain the resistive fraction constant and increase the reactive power from 0 to 800 VAR. Moreover, the simulation setup is analyzed for three different excitation capacitance values (19, 20, 21 mF). The results show that as the inductive load increases, the
For the selected PAT unit, Ns ¼ 51 [m, m3/s] and Np ¼ 186 [m, kW]. Based on these low specific speed numbers, the expected PAT performance can also be calculated following the procedure described in Ref. [13]. This results in a PAT design bep of HT 3 bep ¼ 12.5 m, QT bep ¼ 0.036 m /s, which gives a 5% head and 3% discrepancy between both prediction methods. The models use empirical equations which, as previously mentioned, can have a 20% difference to the actual performance. If after installation the power output differs considerably from the calculated value, the installer can reduce the impeller’s diameter to slightly increase the efficiency of the operation [16]. 3.5. Induction motor as generator As for the electrical component, the motor is a Grundfos 100LC with PN ¼ 3 kWel, p ¼ 4 poles, f ¼ 50 Hz, V ¼ 3 220–240D Volts, cosj ¼ 0.77, IFULL LOAD ¼ 12.4 A, hFULL LOAD ¼ 0.87, and IP55 rating. Thus, considering the mechanical and electrical efficiency, a realistic system efficiency at generation point is hPAT ¼ 0.63, which results in P ¼ 1.9 kWel. The process of using the asynchronous motor as generator (MAG) requires the induction of an excitation
Fig. 3. Frequency during start-up.
M. Arriaga / Renewable Energy 35 (2010) 1109–1115
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system frequency raises (Fig. 4). Nevertheless, this effect can be compensated with the magnitude of the excitation capacitors. In addition, the addition of an induction load affects the system’s voltage; which tends to increase its value and this load is likely to destabilize the voltage (Fig. 5). This issue can also have a negative effect on the system’s stability during the transient stages (e.g. start-up of the refrigerator compressor). 3.6. Civil works design The civil structure design followed is outlined by Maher [6] and head loss calculation, hf, is a standard procedures described by Penche [23]. The civil structures are designed using masonry techniques and sandy soil to keep the civil works costs low. The weir has an approximate dimension of 1 m high, 0.7 m base, 0.4 m top, and 2.5 m long. The canal has a rectangular shape (b ¼ 0.5 m, y ¼ 0.4 m) and approximately 200 m long (hf1 ¼ 0.1 m). Since the flow is only 35 l/s, a small forebay with depth of 1 m is enough for the system. The best option for the penstock is a low pressure PVC pipe for its easy and fast installation, but its availability and price for D > 203 mm (8 in) is the main restriction. The mesh is calculated based on the 0.025 m closed impeller cavities of the pump (hf2 ¼ 0.1 m). The penstock is approximated to 152 mm (6 in) diameter PVC pipe with three elbows (hf3 ¼ 0.8 m). Additionally, since the PAT inlet is 80 mm, a smooth – low transition – contraction is required to avoid a significant head loss in the system (hf4 ¼ 0.1 m). The PAT outlet is 100 mm and a diffuser, to reach a 200 mm diameter, needs to be installed to reduce losses (hf5 ¼ 0.2 m). Finally, a gate valve is used to regulate the incoming flow rate to the PAT (hf6 ¼ 0.1 m). These results in hf ¼ 1.4 m and a net head of HN ¼ 10.6 m. 4. Cost estimation The costs are divided in energy generation equipment (EGE), civil works (CW), and energy distribution costs. The EGE includes the PAT, the IGC controller and the related electrical equipment; which adds up to 2545USD. As a comparison point, the quote for a Vietnamese cross flow turbine and controller is 4800USD. Thus, the PAT implementation has an EGE cost reduction of 53%. The CW costs include the weir, canal, forebay, penstock, and PAT foundation material; which mainly reflects the cost of cement and
Fig. 4. Induction load effect in frequency.
Fig. 5. Induction load effect in voltage.
PVC pipe. The following prices are based on the information obtained from the markets in Vientiane and considering that the weir, canal, and forebay are build with masonry techniques and sand soil to lower their cost. The estimated costs for the cement required for the weir, canal, and forebay are 500USD, 800USD, and 200USD, respectively. The price for the penstock and required PVC valves is 520USD. The foundation to install in the power house is estimated in 400USD. Additionally, the cost of the electrical transmission line from the power house to the NGO office is estimated in 982USD and the cost for spare parts is 263USD. Therefore, the total estimated cost for the CW and transmission line is 3665USD. The CW does not include the costs for labor and equipment transportation since these costs vary depending on each project. Usually the rural electrification projects involve the communities during the construction phase [4,5], which besides social benefits, it also reduces the labor costs. Table 1 summarizes the EGE and CW costs for the proposed project and compares it to appropriate alternatives for energy generation. The available options to consider are the Vietnamese turbine and a rough calculation of 2 kWp of PV panels. The CW costs are maintained equal in both hydro-power cases for ease of comparison. Both PAT scenarios have the lowest installation cost (3105USD/kW – PAT and 4235USD/kW – Turbine); while the PV approach has the highest investment cost. Still, the latter technology does not consider the incurred costs of a higher capacity battery bank for the village evening load (mini-grid case). Since, the objective is to electrify a village of 86 households, the distribution costs are a significant expenditure of the project. The project considers two strategies for providing energy to the households, the battery/lantern and mini-grid approach. The battery-lantern approach includes the costs of the battery charging unit [19] and 86 lanterns for the village. The mini-grid includes the battery bank, the battery inverter, and the distribution grid. Table 2 itemizes the costs of each alternative and shows that, as expected, the cost for the mini-grid alternative is higher than the battery/ lantern installation (58% higher). However, the mini-grid could be justified from a social perspective and that the households would have a continuous supply of electricity. As a note, financial indexes are not calculated for any of the presented options since the project relies completely on donation and the financial aspect is not a priority nor an indicator. The operation and maintenance (O&M) must be supported locally and the village/NGO is responsible of establishing an
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Table 1 Energy generation equipment and civil works costs. Equipment Capacity EGE CW þ (USD) Transm.Line (USD) Proposed PAT V. turbine PV panels
2.0 kWel 2.0 kWel 2.0 kWp
EGE þ CW (USD)
EGE (USD/ kW)
EGE þ CW (USD/kW)
2,545
3,665
6,210
1,275
3,105
4,800
3,665
8,465
2,400
4,235
9,000
–
9,000
4,500
4,500
electrical fee that can maintain the system. The annual O&M costs are estimated to be 350USD and 300USD for the battery charging and mini-grid, respectively. Thus, for the highest cost, the monthly expense is approximately 30USD which can be evenly split between the households and the office. However, even if this is not the case and all the households are involved, the breakeven tariff for the O&M costs is 0.35USD/month$household, which is less than what the villagers are currently paying. The tariff should be set higher since, as mentioned in section II.B, currently the village pays more for candles (4.7USD/month) and kerosene lamps (2.24USD/ month). From previous experience, increasing the tariff at a later date (after implementation) does not receive any village support. 5. Current status and social issues As for the installation timeframe, once the mechanical and electrical equipment arrives to Vientiane, the project, including the civil works, mechanical, and electrical installation, can be implemented in two weeks (one extra week, if the mini-grid is installed). The labor can be provided by the villagers and two Vientiane based technicians. As previously mentioned, one of the objectives of the project is to increase the technical skills of the local technicians to be able to reproduce the project. The full project implementation, including the social structure, was supposed to be completed during October 2008 which would have allowed for sufficient time to setup the technical and social structures required to operate the system. However, due to the unnecessary length of the negotiation process between various stakeholders, the funds could not be secured for the project. Nevertheless, this economic issue could have been solved by requesting funds from other sources. However, pushing the implementation to fit within a narrow timeframe in order to just complete the project is a mistake and a total deviation from the main purpose of providing a long-term electrical service to the village. If the social structure is not present and the appropriate time cannot be spent at the village to setup a solid social/payment structure, the system is likely to be misused/fail and generate tension in the village [2]. Furthermore, the time constraint will certainly have a negative effect on the O&M training which considerably shortens the expected lifetime of the system. In Laos,
several sites were visited in which the lack of training, maintenance tools, and spare parts have degraded full quality mini-hydro schemes to efficiencies of less than 50%. This is likely to happen to this project if the appropriate time is not taken for its technical and, especially, social implementation. 6. Conclusion This paper presented the concept of Pump as Turbine as a viable technical and economical alternative for the further pico-hydro development in the Lao PDR. Given the availability of the appropriate hydro resource, this non-conventional approach can be implemented to electrify isolated communities of 40–500 people with solely hydro power. The paper proves that this method can provide a long-term reproducible system for communities where pico-hydro propeller turbines are not sufficient and proper turbines are significantly more expensive – 3–12 kWel. In addition, the paper presents the feasibility study, design, and analysis of a 2 kWel project in the Xiagnabouli province. The highlights of the proposed project are: the high confidence level in the predicted power output due to the comparison of two methodologies to obtain the PAT curves, the generation equipment cost reduction of 53% when compared to the crossflow turbine approach, the potential implementation of the project by local staff and villagers, and low operation and maintenance costs that can be covered by a reasonable tariff for the villagers. In addition, the simulation of the Motor as Generator presented a positive approach to predict the value of the excitation capacitance required, but most important, the effect of the induction loads in the system behavior. From a practical perspective, it is important to have the induction loads present during the installation phase. This assures that the inductive loads are properly compensated and that the frequency and voltage are maintained between appropriate levels. From a social perspective, the paper also highlights the complicated interactions required to build a social structure and implement the project; especially, when the project timeline is affected by a complex negotiation process. However, it is also pointed out that rushing activities to just complete the project is a mistake and a deviation from the main purpose of providing a long-term electrical service for the village. The future work of this project is obviously the implementation phase with the appropriate training and tariff payment setting. Besides the benefits to the village, the completion of the project will help in further fine-tuning the mechanical operational prediction and the electrical model simulation. Acknowledgements The author would like to acknowledge Dr. Hans Bludszuweit, University of Zaragoza, for proof reading the article. References
Table 2 Total project cost including distribution system. System Part
Battery/ Lantern (USD)
%
Mini-grida (USD)
%
Elec. Gen. Equip. CW þ transmission Batt. Bank/Inverter Distribution grid Batt. Lantern
2,545 3,665 – – 9,295
16% 24% – – 60%
2,545 3,665 10,256 8,017 –
10% 15% 42% 33% –
Total (USD) Total (USD/kW)
15,505 7,752
100%
24,483 12,241
100%
Bold values represent the addition of the above components. a Not included 50USD/household – connection fee.
[1] National Statistics Centre. Chapter 8: housing characteristic. National Statistics Centre of the Lao PDR [Online] Available: http://www.nsc.gov.la/Products/ Populationcensus2005/PopulationCensus2005_chapter8.htm; 2007, Nov 1. [2] Chambers R. Rural development – putting the last first. Essex, England: Prentice Hall; 1983. [3] Leechuefoung P. National Human Development Report Lao PDR – Export of Electricity. Vientiane: United Nations Development Program; 2006. [4] Greacen C. 3 W pump as turbine microhydro at Mae Wei village, Tak province. Palang Thai [Online] Available: http://palangthai.blogspot.com/2008/02/3-kwpump-as-turbine-microhydro-at-mae.html; 2008, Feb 1. [5] Maher P. Design and implementation of a 2.2 kW pico hydro serving 110 households. Micro Hydro Centre – Nottingham Trent University, [Online] Available from: http://www.eee.nottingham.ac.uk/picohydro/documents.html 2002, Feb 7. [6] Maher P, Smith N. Pico hydro for village power – a practical manual for schemes up to 5 W in hilly areas. Micro Hydro Centre – Nottingham Trent
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University, [Online] Available from: http://www.eee.nottingham.ac.uk/ picohydro/documents.html 2001, May 1. Maher P, Smith N, Williams A. Assessment of pico hydro as an option for offgrid electrification in Kenya" Micro hydro Centre – Nottingham Trent University; 2003. pp. 1357–69. Rijsenbeek W. Pico hydro systems in Vietnam. Rural Energy Supply Models [Online] Available: http://resum.ises.org/cgi-bin/resum/resum. py?showproject&PHVietnam; 2001. Smith N, Ranjitkar G. Nepal case study. Pico hydro for rural electrification [Online] Available: http://www.eee.nottingham.ac.uk/picohydro/documents. html; 2000, Mar 1. Thoma D, Kittredge CP. Centrifugal pumps operated under abnormal conditions. Power 1931:881–4. Thode HW, Azbill DC. Typical applications of induction generators and control system considerations. IEEE Transactions on Industry Applications November/ December, 1984;1A-20(6). Isb as¸oiu EC, Bucur DM, Ghergu CM, Tıˆral a NO. "Using standard pumps as turbines. International Group for Hydraulic Efficiency Measurement 2008. Derakhshan S, Nourbakhsh A. Experimental study of characteristics curves of centrifugal pumps working as turbines in different specific speeds. Experimental Thermal and Fluid Science 2008;32:800–7. Gu¨lich JF. "Centrifugal pumps". Turbine operations general characteristics. s.l. Springer; 2007.
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[15] Rawal S, Kshirsagar JT. Numerical simulation on a pump operating in a turbine mode. Texas A&M University System, Turbomachinery Laboratory; 2007. [16] Williams A. Pump as turbines – a user’s guide. 2nd ed. Warwickshire: ITDG Publishing; 2003. [17] M. Smits, ‘‘Neglected decentralized rural electricity production in the Lao PDR – A technography of pico-hydropower analyzed in a political ecology framework’’. M.Sc. Thesis, Department of Social Sciences, Wageningen Univ., Wageningen, Netherlands; 2008. [18] A. Schroeter, S. Martin. Profitable and affordable energy services for remote areas in Lao PDR: private – public partnership as mutual leverage for hybrid village grids in areas off the national grid, 4th European Conference of PVhybrid systems and Mini-grids, Athens. 2008. [19] Schroeter A, Brette JB, Disanayake N. Solar recharging stations – selling hours of solar lighting in the evenings. Energy Concept Papers [Online] Available: http://www.sunlabob.com/en/energy-reports.html 2008, May 1. [20] M. Smits. Pre-feasibility study on pico-hydro and water mills potentials in Xaignabouri district. Vientiane, Lao PDR, unpublished. [21] Ramos H, Borga A. Pumps yielding power. International Group for Hydraulic Efficiency Measurement 2000;10. 04. [22] Smith N. Motors as generators for micro-hydro power. 2nd ed. Warwickshire: Practical Action; 2008. [23] Penche C. Guide on how to develop a small hydropower plant. European Small Hydropower Association [Online] Available: http://www.esha.be/index. php?id¼39; 1998.
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Technical Note
Pump as turbine – A pico-hydro alternative in Lao People’s Democratic Republic Mariano Arriaga* Instituto de Investigaciones Ele´ctricas, Gerencia de Energı´as No Convencionales (Electrical Research Institute, Non-conventional Energy Division), Calle Reforma No. 113, Col. Palmira, 62490, Cuernavaca, Morelos, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 March 2009 Accepted 25 August 2009 Available online 27 November 2009
This paper presents the pico-hydro development status in Lao PDR and introduces the Pump as Turbine (PAT) concept as an alternative for isolated communities (40–500 people). The intention is to provide a long-term reproducible system for communities where pico-hydro propeller turbines are insufficient and proper turbines are expensive. This approach presents a high quality and cost-effective solution for rural electrification which can be installed, commissioned, and maintained by local staff and villagers. Furthermore, a 2 kWel PAT-scheme is proposed for a community in the Xiagnabouli province and considers power generation alternatives, sizing, asynchronous motor simulation, civil works, cost estimation, and social aspects. Ó 2009 Published by Elsevier Ltd.
Keywords: Asynchronous machine simulation Lao PDR Pico-hydro Pump as turbine Renewable energy rural electrification
1. Introduction The Lao People’s Democratic Republic is a land-locked country located in South East Asia. In 2007, Laos had a population of 5.8 million people with a Gross Net Income per Capita of 580USD; which the World Bank classifies as a low income country. As a result, the country has heavily relied over the years on foreign aid for its development. In 2005, the Lao National Statistics Centre estimated that 50% of the households had access to the electrical national grid, 10% had electrical power through their own generator or car battery, and 40% had no access to any electrical source. The electrification level for cities and other densely populated areas is 90%, whereas small isolated communities have a considerably lower level. Thus, only 43% of the rural population with road access has electricity and 11% where no-road access exists [1]. The latter category is not likely to increase its electricity access in the coming years; since, as discussed in Ref. [2] rural poverty is remote and heightened when there is no road access. These percentages need to be taken carefully due to two main reasons. Firstly, the lack of specification for pico-hydro, solar home systems (SHS), diesel engines, or any other source of electricity. To date there is no official/reliable count for such systems in the country. Secondly, the statistics do not consider electrical supply reliability or quality level. In a surveyed community, the voltage variation is between 140 and 250 V and the * Cerro de San Andres 239, Campestre Churubusco, Coyoacan, Mexico City 04200, Mexico. Tel.:þ52 55 5549 5356, þ777 362 3811x7253. E-mail addresses: [email protected], [email protected] 0960-1481/$ – see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.renene.2009.08.022
frequency variation between 38 and 55 Hz due to the lack of maintenance and/or seasonal rainfall variation. In Laos, 97.5% of the electricity generation comes from hydro power and the country is currently increasing its capacity, mostly driven by electrical exports to Thailand and China. The total untapped planned capacity for 2006–2011 is 4013 MW and 2226 MW from 2011 to 2020 [3]. These hydro plans are dealt with at the macro-economic level and do not consider off-grid/small communities where electrical generation and/or grid extension costs deters any feasible commercial approaches.
1.1. Pico-hydro technology Pico-hydro schemes (<5 kW) have been successfully implemented in remote regions in the world [4–8]. Their common objective is to provide a cost effective and simplified alternative to supply electricity to areas that are relatively far from the electrical grid. The types of turbines and generators vary depending on the local conditions, budget, and equipment availability. For high water head (HN > 30 m) areas, a common approach is to locally manufacture Pelton turbines [7,9]. However, Laos’ topography has limited areas where the head meets this requirement. For low head sites (<2–3 m), run of the river schemes include the massproduced Pico-hydro propeller (PHP) turbine coupled to a synchronous permanent magnet generator [8]. These PHP units are found in markets and hardware stores and their capacity ranges from 0.2 kWel to 3 kWel and their price ranges from 35USD to 300USD, respectively. In addition, there are mini-hydro turbine manufacturers outside Laos
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which provide a high quality and efficient unit which can be easily installed and maintained. Their manufacturing costs are frequently higher than all the previously mentioned approaches since currently there are no local businesses in the country. 1.1.1. Pump as turbine (PAT) Thoma presents the first published work regarding pumps running in abnormal conditions [10]; such work is retaken by Thode to outline the use of standard pumps running in reverse for electrical power generation [11]. The paper states the advantages of the PAT and remarks that radial, mixed and axial centrifugal pumps can be successfully operated in reverse when a design review of the pump characteristics is done to ensure proper operation under s¸oiu presents a brief description turbine operating conditions. Isba of PAT operation and related equations; however, no experimental data is provided to validate the model [12]. Derakhshan presents a thorough theoretical model for mixed flow PATs, in which several dimensioning parameters of the pump are required to calculate the expected efficiency [13]. The results presented show a precise model to characterize low specific speed (Ns) turbines 15–55 rpm [m, m3/s]. Similarly, Gu¨lich presents a complementary framework for units ranging from 10 to 200 rpm [m, m3/s] and the turbine/ pump operation based on the unit’s specific speed (Ns), flow rate (Q), head (H), and Torque (M) [14]. In Section 3.3, these theoretical frameworks are applied to calculate the expected operational point and efficiency of the selected PAT. Additionally, Rawal presents a Computational Fluid Dynamics (CFD) simulation for a mixed flow PAT (Ns ¼ 94 rpm [m, m3/s]) However, at this stage the results cannot be generalized and still require further modeling [15]. From a practical perspective, Williams presents a remarkable guide for sizing PAT units, the empirical equations to calculate the expected output, and a brief troubleshooting guide [16]. The guide also mentions the alternative of reducing the impeller’s diameter to reach a closer to optimal operating point for the pump running in reverse. Finally, Maher and Greacen present successful PAT projects used for rural electrification in developing countries [4,5]. 1.2. Off-grid power in the Lao PDR Smits makes a conservative estimation of the number of PHP turbines within Laos, based on empirical data [17]. It is estimated that 60,000 units are installed, mostly in the Northern provinces, providing electricity to 90,000 people. Though, there are commonly identified failure causes including core demagnetization, burn out windings, and worn upper/lower bearings. This latter failure mode is the most widely found and, in particular, the lower bearing. The PHP units are a respectable available option when connecting one household these units are rarely used to connect numerous homes. Recently, a hybrid system was installed in the Xiangkhoung province which consists of a 12.5 kWel mini-hydro unit, 1.5 kWp PV panels, and a 10 kWel diesel genset [18]. From a technical perspective, the system successfully supplies a 100 household village with a constant and reliable source of electricity. However, the social structure and electrical consumption forecast have not developed as expected. The system has an average use of 10 kWh/ day and it is estimated that there is only a 4–5 kWel peak power which results in a power utilization of 20% during the peak hours. Thus, from an economical perspective the project cannot be replicated in similar conditions. In summary, remote rural electrification in Laos requires approaches that cover several technical issues, such as cost effectiveness, repeatability, easy installation and maintenance, local availability, and reliability; which, for certain sites, can be accomplished by a PAT scope. Furthermore, from a technical and economical view, this alternative bridges the gap between the PHP
and cross flow turbines. In chapter II, this paper estimates the potential in Laos for PAT installations. Chapter III presents the natural resource assessment, system design, PAT selection, energy source, energy consumption profile, and induction motor simulation. Chapter IV outlines the estimated cost for the civil, electrical, and mechanical works required and chapter V gives a general overview of the social implications and long-term maintenance. 2. Pico-hydro villages 2.1. Target village estimation for PAT units The following calculations use the electrical consumption data acquired from the hybrid system: E ¼ 0.19 kWh/(day$household) and Ppeak ¼ 75 Wel/house. The PAT electrical power range considered is 1–12 kWel at the generation point. The lower limit is set by the available PHP turbines and the upper limit is set by the existing power controller capacity. Assuming a design factor of 0.85, the size of the village that can be electrified ranges from 10 to 140 households (40–500 people) solely using a PAT system. The main issue involves covering the electrical power peak and not the energy demand which can be overcome with certain design modifications. The first alternative includes a mini-grid with a central battery system. The second alternative is to use a battery charging station [6] which has economic advantages. Currently, there is an ongoing effort to implement a solar battery-lantern project in Laos [19] which at this stage, aims to define the consumer patterns and technical specifications for the system. Next, the design, simulation, and dimensioning for a specific PAT project is presented including the discussed design alternatives. 2.2. Specific site selection Doikao is a small village with 86 households located in the Xiagnabouli province (19 230 N 101710 E). The village is considered poor with an average household income of 180USD/year (0.5USD/ day) which seldom covers the basic needs – for reference a liter of potable water costs 0.2USD. The current sources of lighting are candles 4.7USD/month and kerosene lamps which, on average, consume 1.5 l a month and costs 2.24USD/month. However, there is a significant disparity between this cost and what they are willing to pay for electricity: two lights 0.23–0.59USD/month and four lights/CD player 1.17USD/month [20]. 3. Renewable energy system sizing 3.1. Natural resources in Doikao The potential natural resources for power generation are solar, wind, and hydro power. The solar irradiation is approximately 4.9 kWh/(m2$day). As for the wind resource, the northern areas have poor wind velocity (<5.5 m/s) which cannot be utilized as a significant source of power generation. As for the hydro resource, the village has a stream located 500 m away from the NGO office. A preliminary survey by Smits during the dry season measured a flow rate of Q ¼ 35 l/s and a gross head HG ¼ 12 m [20]. A single measurement for the river flow rate is not ideal; however, detailed information is complex to obtain since the site is not easily accessible. Still, using the dry season flow rate for a PAT design assures that the system’s output can be guaranteed for the full year. 3.2. Design phase The objective is to supply electrical power for lighting of the Doikao village and an NGO office located in the village. The most
M. Arriaga / Renewable Energy 35 (2010) 1109–1115
viable alternatives for electrical power generation are solar and/or hydro power. The office is intended to be electrified directly by the pico-hydro system which loads include lights, laptops, refrigerators and communication equipment. In addition, the village can potentially be supplied by three alternatives: SHS, battery charging unit or a mini-grid system. 3.2.1. Option 1 – pico-hydro unit and SHS A SHS of approximately 20–30 Wp can supply each household with lighting. The advantage of this system is the simplicity of the ownership process and familiarity of the installation procedure in Laos. Nevertheless, this approach has a significantly low hydro system usage (51%); since approximately 22 kWh/day are directed to the dump load and 23.5 kWh/day are used by the actual loads. 3.2.2. Option 2 – pico hydro and battery charging The excess energy directed to the dump load can be used to run a battery charging station (600 Wel) to supply electric power to the village. For example, considering that the charging station runs for 10 h/day, one 12V/10Ah battery/household, and a system performance of 50%, the unit can potentially charge the equivalent of 25 households/day – 30% of the village. The main advantage is that no additional equipment is required. Each household would be responsible for its battery and a fee could be paid every time the battery is charged which in turn would generate income. From an energy point of view, this configuration results in a 61% hydro system usage, with 28 kWh/day directed to the loads and 17.5 kWh/ day directed to the dump load. 3.2.3. Option 3 – pico-hydro mini-grid system An electrical mini-grid scenario can supply power to the office and village using the pico hydro and a centralized battery bank. The office loads are kept unchanged and an average of 25 Wp/household is allocated. Furthermore, if an estimated battery power roundtrip efficiency of 73% is used, then 6.3 kWh/day are needed to cover the energy night peak which is lower than the 16.2 kWh/day energy surplus (dump load). Considering the 6.3 kWh/day, 1.9 days of autonomy, and 40% depth of discharge, a battery bank of 30 kWh (24V/1 250Ah) can be installed to cope with the demand peak. Fig. 1 presents the mean power profile for the three scenarios considered.
3.3. Pico-hydro turbine options 3.3.1. Pico-hydro propeller turbine The largest available turbine is 3.0 kWel which assuming 45% efficiency has an electrical output of 1.35 kWel at the generation point. Furthermore, the quality of the turbines is dubious and most of them present machined areas with visibly poor surface finish which, obviously, increases wear and decreases efficiency. 3.3.2. Cross flow turbine The main advantage is the flexibility of running the unit at different flow rates which, increases the power during the rainy season. Thus, there is currently no turbine manufacturer in Laos; which means that the construction and installation of the turbine is done by foreign staff. 3.3.3. Pump as turbine Besides the previously mentioned advantages, the system can be designed and installed by local staff, the pump distributors are locally available assuring spare parts on hand, and most importantly, the maintenance can be done by the villagers. However, the PAT approach has two main drawbacks: the fixed operational speed and the limited accuracy in the performance prediction. As a result, since the main objective is to provide a high quality, cost effective, long-term solution, the PAT alternative is selected as the most viable option.
3.4. Pump as turbine selection The design criteria for the hydro scheme is based on a flow rate of Q ¼ 0.035 m3/s and a gross head of HG ¼ 12 m; which is to generate electricity at 230/400 V–50 Hz. As for the available pumps, Laos has two major pump manufacturers: Kirloskar and Grundfos. The objective is to convert the pump curves and best efficiency points (bep) to its turbine equivalent. The following formulas are used to calculate the flow rate and head when running the pump on reverse mode [16].
Qt ¼
nt Qbep $ np h0:8 max
Ht ¼
Fig. 1. Daily energy profile.
1111
nt np
2
Hbep $
h1:2 max
(1)
(2)
where, Qbep and Hbep are the bep of the pump, hmax is the pump’s maximum efficiency, and np and nt are the rotational speed (rpm) when running as pump and turbine, respectively. These equations are based on the standard pump Affinity Laws. Since, these equations have an empirical nature; the actual PAT performance can potentially differ by 20% [16]. The system curves are calculated based on the commonly used Manning equation for head losses, the pump curves are taken from the manufacturers’ specifications and the PAT bep are calculated using (1) and (2) at maximum and 7% reduced efficiency and the design range is set at 0.035–0.040 m3/s. As a result, from the prospective PAT analyzed, PAT NB 80-160/175 and NB 100-250/236 have similar operational curves and are the closest to the design criteria (Fig. 2). Still, PAT NB 80-160/175 is selected for economic reasons. The characteristics of the pump are HP bep ¼ 8 m, QP bep ¼ 0.028 m3/s, hmax ¼ 80%, n ¼ 1445 rpm, p ¼ 4 poles, and slip ¼ 0.037. Based on these specifications in Eqs. (1) and (2), the corresponding parameters when running as turbine are HT bep ¼ 12 m, QT 3 bep ¼ 0.035 m /s, and n ¼ 1555 rpm, if the manufacturers efficiency is considered. However, if a more realistic efficiency is used
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current in the motor windings. Smith describes a methodology to calculate the required capacitance and its connection to obtain a stable one-phase operation by using a C–2C delta connection [22]. These procedure results in a self-excitation capacitance of C ¼ 23 mF and the expected stable operation point at P ¼ 1.95 kWel.
Fig. 2. PAT curves with bep at rated and decreased efficiency.
(h ¼ 73%), then the corresponding values are HT bep ¼ 13.4 m, QT m3/s. In addition, Derakhshan and Ramos, among others, have designed estimation methods based on the pump specific speed [13,21]. Each author uses a different definition for specific speed depending upon the parameters used for the calculation. The following are the respective equations used.
bep ¼ 0.038
NS ¼ Nm; m3 =s ¼
NP ¼ Nðm;kWÞ
np $
qffiffiffiffiffiffiffiffiffiffi Qbep
0:75 Hbep
pffiffiffi nP $ P ¼ 1:25 Hbep
(3)
(4)
3.5.1. Electrical system simulation The MAG model is built in Matlab/Simulink using the SimPower Systems Toolbox to represent a self-excited three phase squirrelcage asynchronous generator. The further motor parameters required are the stator and rotor resistance Rr ¼ 1.36 U, inductance Lls ¼ 5.8 mH, and inertia J ¼ 0.076 kg m2. The asynchronous machine power source is the mechanical torque, T ¼ 14.7 N m, provided by the PAT taking into account its mechanical efficiency, h ¼ 0.8, and rotor speed, n ¼ 1555 rpm, required when running as a generator. The excitation capacitance must be induced to create the magnetization current; in this case, a three phase capacitor bank model is used. Finally, a three phase maximum resistance of 2.4 kWel is connected as a load. This simulation represents a realistic model for an induction MAG; however, as any model, there are certain assumptions and limitations. The motor inductance values used are of a similar motor; since the manufacturer’s information is not available. In addition, the asynchronous machine model in Simulink is already ‘‘internally’’ wired as a wye connection which avoids the possibility of a C–2C arrangement to attain a one-phase stable system. However, this step is more important from a system-stability than from the excitation capacitance and induction load perspectives. The simulation time is intended to cover the start-up effect of the electrical system, t ¼ 1.2 s. The results from the simulation show that as the excitation capacitance increases, the frequency proportionally decreases and a similar effect happens with the efficiency (Fig. 3). Thus, during implementation, it is advisable to avoid over-compensation since this decreases the frequency, motor temperature, efficiency, and voltage. The previous results use a solely resistive load; then again, the system is intended to run both, resistive and inductive loads. The procedure followed to analyze the behavior of inductive loads is to maintain the resistive fraction constant and increase the reactive power from 0 to 800 VAR. Moreover, the simulation setup is analyzed for three different excitation capacitance values (19, 20, 21 mF). The results show that as the inductive load increases, the
For the selected PAT unit, Ns ¼ 51 [m, m3/s] and Np ¼ 186 [m, kW]. Based on these low specific speed numbers, the expected PAT performance can also be calculated following the procedure described in Ref. [13]. This results in a PAT design bep of HT 3 bep ¼ 12.5 m, QT bep ¼ 0.036 m /s, which gives a 5% head and 3% discrepancy between both prediction methods. The models use empirical equations which, as previously mentioned, can have a 20% difference to the actual performance. If after installation the power output differs considerably from the calculated value, the installer can reduce the impeller’s diameter to slightly increase the efficiency of the operation [16]. 3.5. Induction motor as generator As for the electrical component, the motor is a Grundfos 100LC with PN ¼ 3 kWel, p ¼ 4 poles, f ¼ 50 Hz, V ¼ 3 220–240D Volts, cosj ¼ 0.77, IFULL LOAD ¼ 12.4 A, hFULL LOAD ¼ 0.87, and IP55 rating. Thus, considering the mechanical and electrical efficiency, a realistic system efficiency at generation point is hPAT ¼ 0.63, which results in P ¼ 1.9 kWel. The process of using the asynchronous motor as generator (MAG) requires the induction of an excitation
Fig. 3. Frequency during start-up.
M. Arriaga / Renewable Energy 35 (2010) 1109–1115
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system frequency raises (Fig. 4). Nevertheless, this effect can be compensated with the magnitude of the excitation capacitors. In addition, the addition of an induction load affects the system’s voltage; which tends to increase its value and this load is likely to destabilize the voltage (Fig. 5). This issue can also have a negative effect on the system’s stability during the transient stages (e.g. start-up of the refrigerator compressor). 3.6. Civil works design The civil structure design followed is outlined by Maher [6] and head loss calculation, hf, is a standard procedures described by Penche [23]. The civil structures are designed using masonry techniques and sandy soil to keep the civil works costs low. The weir has an approximate dimension of 1 m high, 0.7 m base, 0.4 m top, and 2.5 m long. The canal has a rectangular shape (b ¼ 0.5 m, y ¼ 0.4 m) and approximately 200 m long (hf1 ¼ 0.1 m). Since the flow is only 35 l/s, a small forebay with depth of 1 m is enough for the system. The best option for the penstock is a low pressure PVC pipe for its easy and fast installation, but its availability and price for D > 203 mm (8 in) is the main restriction. The mesh is calculated based on the 0.025 m closed impeller cavities of the pump (hf2 ¼ 0.1 m). The penstock is approximated to 152 mm (6 in) diameter PVC pipe with three elbows (hf3 ¼ 0.8 m). Additionally, since the PAT inlet is 80 mm, a smooth – low transition – contraction is required to avoid a significant head loss in the system (hf4 ¼ 0.1 m). The PAT outlet is 100 mm and a diffuser, to reach a 200 mm diameter, needs to be installed to reduce losses (hf5 ¼ 0.2 m). Finally, a gate valve is used to regulate the incoming flow rate to the PAT (hf6 ¼ 0.1 m). These results in hf ¼ 1.4 m and a net head of HN ¼ 10.6 m. 4. Cost estimation The costs are divided in energy generation equipment (EGE), civil works (CW), and energy distribution costs. The EGE includes the PAT, the IGC controller and the related electrical equipment; which adds up to 2545USD. As a comparison point, the quote for a Vietnamese cross flow turbine and controller is 4800USD. Thus, the PAT implementation has an EGE cost reduction of 53%. The CW costs include the weir, canal, forebay, penstock, and PAT foundation material; which mainly reflects the cost of cement and
Fig. 4. Induction load effect in frequency.
Fig. 5. Induction load effect in voltage.
PVC pipe. The following prices are based on the information obtained from the markets in Vientiane and considering that the weir, canal, and forebay are build with masonry techniques and sand soil to lower their cost. The estimated costs for the cement required for the weir, canal, and forebay are 500USD, 800USD, and 200USD, respectively. The price for the penstock and required PVC valves is 520USD. The foundation to install in the power house is estimated in 400USD. Additionally, the cost of the electrical transmission line from the power house to the NGO office is estimated in 982USD and the cost for spare parts is 263USD. Therefore, the total estimated cost for the CW and transmission line is 3665USD. The CW does not include the costs for labor and equipment transportation since these costs vary depending on each project. Usually the rural electrification projects involve the communities during the construction phase [4,5], which besides social benefits, it also reduces the labor costs. Table 1 summarizes the EGE and CW costs for the proposed project and compares it to appropriate alternatives for energy generation. The available options to consider are the Vietnamese turbine and a rough calculation of 2 kWp of PV panels. The CW costs are maintained equal in both hydro-power cases for ease of comparison. Both PAT scenarios have the lowest installation cost (3105USD/kW – PAT and 4235USD/kW – Turbine); while the PV approach has the highest investment cost. Still, the latter technology does not consider the incurred costs of a higher capacity battery bank for the village evening load (mini-grid case). Since, the objective is to electrify a village of 86 households, the distribution costs are a significant expenditure of the project. The project considers two strategies for providing energy to the households, the battery/lantern and mini-grid approach. The battery-lantern approach includes the costs of the battery charging unit [19] and 86 lanterns for the village. The mini-grid includes the battery bank, the battery inverter, and the distribution grid. Table 2 itemizes the costs of each alternative and shows that, as expected, the cost for the mini-grid alternative is higher than the battery/ lantern installation (58% higher). However, the mini-grid could be justified from a social perspective and that the households would have a continuous supply of electricity. As a note, financial indexes are not calculated for any of the presented options since the project relies completely on donation and the financial aspect is not a priority nor an indicator. The operation and maintenance (O&M) must be supported locally and the village/NGO is responsible of establishing an
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Table 1 Energy generation equipment and civil works costs. Equipment Capacity EGE CW þ (USD) Transm.Line (USD) Proposed PAT V. turbine PV panels
2.0 kWel 2.0 kWel 2.0 kWp
EGE þ CW (USD)
EGE (USD/ kW)
EGE þ CW (USD/kW)
2,545
3,665
6,210
1,275
3,105
4,800
3,665
8,465
2,400
4,235
9,000
–
9,000
4,500
4,500
electrical fee that can maintain the system. The annual O&M costs are estimated to be 350USD and 300USD for the battery charging and mini-grid, respectively. Thus, for the highest cost, the monthly expense is approximately 30USD which can be evenly split between the households and the office. However, even if this is not the case and all the households are involved, the breakeven tariff for the O&M costs is 0.35USD/month$household, which is less than what the villagers are currently paying. The tariff should be set higher since, as mentioned in section II.B, currently the village pays more for candles (4.7USD/month) and kerosene lamps (2.24USD/ month). From previous experience, increasing the tariff at a later date (after implementation) does not receive any village support. 5. Current status and social issues As for the installation timeframe, once the mechanical and electrical equipment arrives to Vientiane, the project, including the civil works, mechanical, and electrical installation, can be implemented in two weeks (one extra week, if the mini-grid is installed). The labor can be provided by the villagers and two Vientiane based technicians. As previously mentioned, one of the objectives of the project is to increase the technical skills of the local technicians to be able to reproduce the project. The full project implementation, including the social structure, was supposed to be completed during October 2008 which would have allowed for sufficient time to setup the technical and social structures required to operate the system. However, due to the unnecessary length of the negotiation process between various stakeholders, the funds could not be secured for the project. Nevertheless, this economic issue could have been solved by requesting funds from other sources. However, pushing the implementation to fit within a narrow timeframe in order to just complete the project is a mistake and a total deviation from the main purpose of providing a long-term electrical service to the village. If the social structure is not present and the appropriate time cannot be spent at the village to setup a solid social/payment structure, the system is likely to be misused/fail and generate tension in the village [2]. Furthermore, the time constraint will certainly have a negative effect on the O&M training which considerably shortens the expected lifetime of the system. In Laos,
several sites were visited in which the lack of training, maintenance tools, and spare parts have degraded full quality mini-hydro schemes to efficiencies of less than 50%. This is likely to happen to this project if the appropriate time is not taken for its technical and, especially, social implementation. 6. Conclusion This paper presented the concept of Pump as Turbine as a viable technical and economical alternative for the further pico-hydro development in the Lao PDR. Given the availability of the appropriate hydro resource, this non-conventional approach can be implemented to electrify isolated communities of 40–500 people with solely hydro power. The paper proves that this method can provide a long-term reproducible system for communities where pico-hydro propeller turbines are not sufficient and proper turbines are significantly more expensive – 3–12 kWel. In addition, the paper presents the feasibility study, design, and analysis of a 2 kWel project in the Xiagnabouli province. The highlights of the proposed project are: the high confidence level in the predicted power output due to the comparison of two methodologies to obtain the PAT curves, the generation equipment cost reduction of 53% when compared to the crossflow turbine approach, the potential implementation of the project by local staff and villagers, and low operation and maintenance costs that can be covered by a reasonable tariff for the villagers. In addition, the simulation of the Motor as Generator presented a positive approach to predict the value of the excitation capacitance required, but most important, the effect of the induction loads in the system behavior. From a practical perspective, it is important to have the induction loads present during the installation phase. This assures that the inductive loads are properly compensated and that the frequency and voltage are maintained between appropriate levels. From a social perspective, the paper also highlights the complicated interactions required to build a social structure and implement the project; especially, when the project timeline is affected by a complex negotiation process. However, it is also pointed out that rushing activities to just complete the project is a mistake and a deviation from the main purpose of providing a long-term electrical service for the village. The future work of this project is obviously the implementation phase with the appropriate training and tariff payment setting. Besides the benefits to the village, the completion of the project will help in further fine-tuning the mechanical operational prediction and the electrical model simulation. Acknowledgements The author would like to acknowledge Dr. Hans Bludszuweit, University of Zaragoza, for proof reading the article. References
Table 2 Total project cost including distribution system. System Part
Battery/ Lantern (USD)
%
Mini-grida (USD)
%
Elec. Gen. Equip. CW þ transmission Batt. Bank/Inverter Distribution grid Batt. Lantern
2,545 3,665 – – 9,295
16% 24% – – 60%
2,545 3,665 10,256 8,017 –
10% 15% 42% 33% –
Total (USD) Total (USD/kW)
15,505 7,752
100%
24,483 12,241
100%
Bold values represent the addition of the above components. a Not included 50USD/household – connection fee.
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