Energy from solar and hydro power

I AM a mechanical engineer and in my over 30 years living in Malaysia, I have been involved with many Tenaga Nasional Bhd power projects. I was employed in TNB in the late 1980s and have since worked for other local companies and am currently a consultant for insurance surveys and accident investigations for power plants in the region.

The current focus is on using renewable energy for power generation to avoid the use of fossil fuels and reduce the carbon footprint of the power generation industry. The renewable energy plants are reliant on the foibles of nature and as such do not have a constant availability and so their use has to be designed to overcome these natural effects of no wind, no sun, etc. My following thoughts offer a potential solution to make the photo voltaic power plant a more practical, reliable form of renewable energy.

Conventional power stations are theoretically located by the costs of three determinants – the cost of the electrical supply to the load centre, the cost of supply and infrastructure of the fuel supply, and the cooling water supply.

The load centre is the industrial and domestic electrical demand for factories and residential housing estates, which are supplied from the power station by a transmission line and substations and the cost of this system will be determined by the land acquisition costs and the distance from the load centre to the power station.

The fuel supply is equally costed, whether gas or oil supplied through a pipeline and pumping stations or coal transported by barges to coastal jetties and transported to the power station by conveyors or trucks.

The quantity of cooling water supplies will depend on the type of power station and the practicability of the potential location of the power station. Cooling water supplies are preferably sourced from the sea or rivers but in the absence of proximity to these sources, the remaining options are cooling towers or air/water heat exchangers.

The costs of these determinants are priced and mathematical optimisation will determine the least cost option of the potential locations. Other considerations may override the purely technical aspects. Such considerations as land acquisition and development areas can change the location of the ideal lowest cost option.

In a country like Malaysia blessed with abundant sunshine, photo voltaic power stations can be located almost anywhere. However, the proximity to a load centre will be a major consideration due to the cost of the transmission line to the load centre. For wind power, similar considerations will apply but Malaysia only receives significant wind prior to storms.

The major disadvantage with the photo voltaic (PV) power supply is its continuity of supply. If there is no sunshine, there is no power supply. For a reliable, constant source of supply from the PV station, a standby power source has to be installed in case of lack of sunshine or if there is to be an electrical supply at night. Another option to maintain continuity of supply would be storage of energy such as a battery system.

Another disadvantage of the PV plant is its power output when compared to a major power station. The largest PV station is around 50MW and Malaysia’s major power stations have capacities well in excess of 1,000MW with generating units of the order of 300-500MW. Thus the matching, balancing of outputs of power stations with smaller stations becomes less easy.

The use of renewable power is a noble aim but having to install extra generation capacity as standby to maintain a continuous electrical supply increases the capital cost of the installation and this lowers the overall efficiency of the electrical generation system with the standby plant idle. If the standby plant or battery storage is included in the overall operating costs, the renewable energy cost will not look so favourable.

A way to overcome this disadvantage would be to install PV power stations next to existing conventional power stations which can be used for the back-up. Or mount the PV cells on the power station buildings. The power station buildings cover a large area which can be utilised.

However, it therefore implies that the existing station is disadvantaged by having a reduced output when the sun is shining to give preferential generation to the PV plant. The conventional power station operating efficiency is thus reduced from its optimum.

However, the arrangement has an advantage – the power export from the PV plant can utilise the transmission lines from the existing station thus reducing the capital cost of the PV plant. This arrangement would provide preferential treatment of the PV plant and in a competitive environment the conventional stations would cry foul unless some compensation was given.

An interesting variation on the above solution would be to locate the PV station next to a hydro power station with a storage dam. TNB usually operates hydro dams to conserve water and maintain the storage quantity of water for emergencies and for peak lopping. The hydro power plant has the advantage of generating electricity within a few minutes. A conventional power station will take a few hours to reach full load depending on the size and type, and its state of readiness. At various times the hydro plant is on standby and not generating. And thus the transmission lines from the hydro plant to the load centres are under-utilised. With the PV plant next to the hydro plant, the transmission lines can be utilised for PV plant output.

A perceived advantage of this arrangement is the supposition that when it is raining, there is no sun and vice versa. When the sun shines the PV plant provides generation and when it is raining the hydro plant can be generating.

The hydro station is usually located in a rural area and the storage dam covers a wide and largely unproductive area. This large expanse of water and surround- ings can be covered with photo voltaic cells to provide the PV power plant output.

A major disadvantage of the photo voltaic method of power generation is that it is only available when the sun shines (or the solar radiation is sufficient). If the renewable energy source were to be considered as a reliable continuous source of electrical energy, it has to be coupled with energy storage or back-up power generation sources. Battery storage is the common solution for the back-up energy source but the quantity of energy storage to back up the PV plant for a length of time would be significantly large.

Another form of energy storage system is a pumped water storage system which consists of two reservoirs – one at a high level and another at a lower level. When there is excess electrical generation, power is used to pump water from the lower level to the higher reservoir. At times when there is a shortfall in generation, water from the higher reservoir is used for generation.

The turbines used are reversible, that is, they are capable of both generation and pumping. This method of pumped storage has been adopted in a number of countries.

With the PV plant coupled with a pumped storage scheme, the plant can be considered as a continuous source of power, negating some of the disadvantages levelled at the solar plant.

Another advantage in locating a PV plant with a hydro station is that some of the electrical auxiliaries will be of similar size to the smaller hydro plants. These plants will have generators in the 50MW range, so step-up transformers and switchgear will be of similar size and can share maintenance procedures and spare parts.

I believe this is a concept worth some further thought.