India aspires to achieve an ambitious target of 175GW of renewable energy by 2022. A large share of this goal — 100GW — is expected to come from solar energy and a relatively smaller share from bio-energy, which is contributing 10GW.

Today, the market is dominated by the photovoltaic (PV) technology, which has an installed capacity of almost 7000 MW. Also worth noticing is the fact that the price of solar PV fell from ₹17 to ₹5 per unit (1 unit = 1 kWh) in the past few years, while the price of solar thermal has remained above ₹10/kWh.

As solar energy is available only a few hours in a day, it is desirable to store it, to be able to dispatch power when required. However, battery storage is expensive and unviable for long-term use, as batteries need to be replaced at least every five years. Hence, PV with battery storage, using current technologies, looks unviable as a long-term solution for dispatchable and reliable power.

Thermal power

Solar thermal, which concentrates sun’s rays to produce heat (steam), is used to drive turbines. This heat energy can be stored and used to generate electricity when there is no sunlight; this makes it cost-effective in comparison with battery storage.

Worldwide, most solar thermal power plants have incorporated thermal storage. Solar thermal with storage facility can be used to overcome intermittency and provide steady power to grids; however, the inclusion of thermal storage increases the capital cost further.

In India, solar thermal technology had limited success in the initial phase of the National Solar Mission owing to challenges in terms of financial closure, sourcing of critical components of the power plant, etc., and this led to a substantial delay in their commissioning. This also resulted in a reduction of the allotted target in the subsequent phases.

Also, the capital cost of solar PV reduced from ₹12.5 crore/MW to ₹5.8 crore/MW, whereas that of solar thermal remained at ₹12 crore/MW. This led to the dominance of PV in the Indian market. Hence, solar thermal has not been able to compete with solar PV.

On the other hand, biomass-based technologies are relatively mature here, with investment and generation costs of about ₹6.5 crore/MW and ₹4.5/unit, respectively. However, biomass plants are also not able to progress due to various reasons such as availability (lack of a well-established supply chain) and variations in costs of biomass.

Mix and make

Considering India must have a mix of both thermal and PV technologies to achieve 100GW, and also to bring down the cost of solar thermal, one of the options is to have thermal storage or to hybridise solar thermal power plants with biomass.

Biomass is predominantly used by combustion and gasification technologies to generate power. It is estimated that 120-150 million tonne of surplus biomass is available in India annually, which has a potential of generating 18GW.

However, the typical capital cost for setting up a biomass-based plant is about ₹6.5 crore/MW, while the benchmark cost of biomass-based power is ₹4.5/kWh. Hence, biomass-based standalone power plants have not been popular in India.

Both solar thermal and biomass technologies work on the same principle. The heat generated is used to produce steam, which is in turn fed into a turbine to generate electricity. Thus, a hybrid of a solar thermal plant and a biomass plant is a viable option. For example, based on the Central Electricity Regulatory Commission guidelines, the capital cost of a 3MW hybrid plant is about ₹30 crore, whereas the cost of a standalone solar thermal plant is about ₹45 crore and that of a biomass-based plant of equal capacity would be about ₹20 crore. The capital cost of a hybrid plant reduces due to the use of common equipment (turbine generator set, cooling systems, etc.).

Though a biomass plant costs less compared to a hybrid one, the latter scores on operational parameters. Operating costs of a biomass plant increase with the rise in fuel requirements. Biomass prices also fluctuate typically between ₹3-6 per kg, or even more.

The availability of biomass and supply chain is also a major issue. Hybrid plants also score because they require considerably lower quantity of biomass as it is used in combination with solar radiation.

A comparison of the tariffs of the power generated from a standalone solar thermal and a hybrid system shows that the costs range from ₹14-16/kWh and ₹8/kWh, respectively. These estimates are based on a normal eight-hour operation considering only solar hours. If the plant runs 24 hours, the cost comes down to ₹5.4/ kWh.

Hybridisation will increase the capacity utilisation factor of a plant threefold, as it will generate electricity from biomass in the absence of sunlight. This will make the plant financially viable.

Also, biomass-based power can help overcome the intermittency without adding thermal storage. A standalone biomass-based power plant requires a continuous supply of biomass in large quantities. Hybridisation with solar thermal will reduce the biomass requirement, and in the case of a temporary shortage of biomass, power production will be curtailed but the plant will remain functional throughout the year.

Some hurdles

However, the hybridisation model has some issues. Securing a continuous supply of biomass still remains a hurdle because there is no provision for a long-term purchase agreement between biomass suppliers and power producers, which makes the price of biomass unpredictable. Another issue is the configuration of plants (i.e., solar and biomass contribution) that can produce electricity at the lowest cost. Regulatory issues such as monitoring the contribution of each source in power generation remain a point of contention.

A 3MW hybrid is being implemented in Bihar as a demonstration project by CSTEP along with Indian and European partners. Currently, there is no regulatory and policy framework in India to address or promote hybridised renewable energy plants.

The learnings from the operations of this power plant will lead to a better understanding of the technical challenges associated with hybrid systems, and can lead to better policy frameworks and tariff pricing mechanisms.

Thirumalai is a research scientist and Singh is an intern at CSTEP, Bengaluru