Themes in this post: Power vs heat | Centralized vs distributed | Grid-connected vs  offgrid | With vs without batteries | Solar thermal

Most of you would have seen solar panels on rooftops or images or videos of large-scale solar farms. They look quite simple. While solar energy systems are indeed simple to operate, they pack a punch when it comes to the diversity of offerings they have.

Read on!

Power vs heat

When we see solar panels, we think of solar power. But power is only of the two types of energy that the sun can provide us. The other – something people in India feel most of the year – is heat! 

Which brings us to the main theme of this chapter, about the two main types of solar energy: Solar Photovoltaic (PV) and Solar Thermal.

You can think of these two in simple terms:

• #HSolar PV = electricity#H when we talk of solar power, usually means solar PV.
• #HSolar thermal = heat#H

Solar PV and solar thermal technologies harness solar energy differently.

Solar PV uses photovoltaic cells to directly convert sunlight into electricity, which can then power anything from a small solar lantern to large factories.

In contrast, solar thermal systems generate heat by capturing it from sunlight. This heat can be used for residential applications such as solar water heaters or industrial applications for process heating or drying. When solar thermal is used to generate a high temperature (400 degrees and higher), the heat can also be used to generate power through what are called concentrated solar power (CSP) systems, but the technology to generate power from solar has not picked up, as we will see in a later section in this chapter.

Centralized vs distributed

Centralized solar power generates large amounts of electricity at one location which is then distributed to others. Distributed solar power generates relatively much less at a site but is consumed at or near the site of generation

Centralized solar farms are large, ground-mounted solar parks that typically feed electricity directly into the power grid. With capacities often reaching into gigawatts, centralized farms like Bhadla Solar Park in Rajasthan (over 2,245 MW) illustrate the scalability of this approach.

Distributed solar power plants, on the other hand, consist of smaller, localized solar installations, such as residential (typically 1-10 kW) and commercial (10 kW to several MW) rooftop systems. Distributed systems offer decentralized power generation, reducing transmission losses and supporting energy independence for homes and businesses.

Centralized farms demand extensive land, usually in rural areas, whereas distributed systems require minimal space, often utilizing existing rooftops in urban settings.

Centralized farms primarily “export” the power generated to the grid supply for supplies to many locations, while distributed systems serve localized energy needs.

Grid-connected vs offgrid

Grid connected solar can use electricity from – and give electricity to – the grid. Offgrid solar depends on just the power generated from the solar panels.

Grid-connected solar systems are tied to the main electricity grid, allowing for the transfer of surplus electricity generated back to the grid and the use of grid power when solar output is low. Large-scale ground mounted solar power plants are almost always connected to the grid. For residential and rooftop solar power plants too, grid connectivity is common, especially where policies allow users to offset electricity costs by selling excess power to the grid. 

Off-grid solar systems operate independently of the main grid, most times using batteries for energy storage to ensure a continuous power supply. These systems are vital in remote or rural areas where grid infrastructure may be unavailable. Solar lanterns and solar water pumps exemplify off-grid solutions, designed for low power consumption and often used in rural and agricultural settings.

Grid-connected systems can balance generation and consumption dynamically, while off-grid systems, because of the lack of the big brother grid to help them in times of scarcity, require careful energy storage and usage planning.

Grid-connected solutions are effective in areas with reliable grid infrastructure, while off-grid systems provide essential access to power in remote regions without grid connections.

With & without batteries

Without batteries – or some other form of storage – it is not possible to use solar power throughout the day.

Solar power plants with battery storage offer enhanced reliability, storing energy generated during the day for use at night or during cloudy conditions. As mentioned earlier, batteries are crucial in off-grid systems and also select grid-connected systems where consistent energy access is needed, particularly for remote, rural locations. Off-grid solar water pumps and lanterns, for instance, rely on batteries to function effectively without constant sunlight.

Solar power plants without batteries are typically found in ground-mounted solar power plants. For example, large-scale solar parks operate without batteries, feeding power directly to the grid for immediate use. Battery-less solar power plants are also found in grid-connected configurations for rooftop solar power plants where the grid itself acts as a backup. 

Batteries add to the reliability and continuous supply of power, but significantly add to the initial investment costs – #Hin some cases as much 100% more#H, and in many cases an increase of about 50% – impacting affordability.

While batteries provide energy independence, they have limited storage capacity and lifespan (lifespan much less than that of the solar panels), making them suitable for small systems but less viable for large-scale installations. However, with the increasing proportion of solar power entering the Indian grid, and with the falling battery prices, one could see batteries becoming commonplace in large scale solar power plants as well.

Solar thermal – the poor cousin

While most of us tend to think that the Indian solar energy story started in 2010, it is not entirely true. What is true is that the Indian solar power sector started in 2010. But solar energy had started contributing much earlier, some three decades prior to this – in the form of heat, and in the shape of solar water heaters.

All forms of solar heating – be it for low temperature water heating or the emerging technologies that can help solar energy generate much higher temperatures – are clubbed under the category called solar thermal.

Let’s look a bit more deeply into solar thermal.

Solar thermal systems capture and convert solar energy into thermal energy (heat).  Common uses include water heating, space heating, and various industrial processes.

Solar photovoltaic (PV) on the other hand converts sunlight into electricity using photovoltaic cells.

Different levels of solar thermal

Solar thermal can provide temperatures from 60 degrees C to over 400 degrees C.

Low-temperature solar thermal systems typically operate below 100 degrees C and are used extensively in residential settings to provide hot water for bathing, cooking, and cleaning. These use fairly simple technology such as flat plate and evacuated tube collectors.

Medium-temperature solar thermal systems typically provide temperatures in the 100-250 degrees C range and are typically used in industries such as food processing, textiles and laundry services for heating processes. The technologies used are a bit more sophisticated than those for low temperature solar and comprise parabolic trough collectors and concentrators. For example, the dairy industry in Gujarat has implemented solar heating for pasteurization, leading to reduced conventional energy costs.

High-temperature solar thermal systems provide temperatures higher than 400°C and are typically used for power generation through what are called Concentrated Solar Power (CSP) Plants. They use technologies that are conceptually similar to medium temperature in that they also concentrate sunlight, but many of these systems enable much higher concentration of the sunlight to derive very high temperatures. Technologies used for such high concentration include Parabolic Dishes, Linear Fresnel Reflectors and Power Towers.

Solar water heaters – low temperature solar thermal

India has witnessed widespread adoption of solar water heaters. As of 2022, India boasts an estimated capacity of 8.6 million square meters of solar collector area, translating to approximately 6.0 GWth.

The Indian government has actively promoted the adoption of solar water heaters through various subsidy schemes and awareness campaigns. These efforts are particularly focused on residential and commercial sectors, aiming to reduce dependency on conventional electricity for water heating.

States like Karnataka and Maharashtra have seen #Hwidespread deployment of solar water heaters#H.

Commercial & industrial applications – medium temperature solar thermal

Industries involved in chemical manufacturing and metal treatment can find strong business case for the use of CST systems.

Commercial applications include use in commercial establishments such as hotels, hospitals, and educational institutions that utilize medium-temperature solar thermal systems for space heating and hot water supply. These applications help in reducing operational costs and enhancing energy efficiency. Hotels employ solar thermal systems to provide hot water for guest services, significantly lowering their electricity consumption and operational expenses. Restaurants can use it for cooking. Prominent examples of cooking using solar thermal include two religious locations – the #HThirumala temple in Tirupathi#H and #HShirdi Sai Baba temple#H.

Industrial applications include process heat in manufacturing in which industries like textiles, food processing, and pharmaceuticals are leveraging medium-temperature solar thermal systems for various heating processes. In states such as Gujarat, textile mills have integrated solar thermal systems to generate steam required for dyeing and finishing processes. The food industry utilizes solar thermal energy for cooking, pasteurization, and drying processes. Pharmaceutical companies use solar thermal systems to maintain precise temperature controls necessary for drug production, ensuring quality while reducing energy expenses.

CST & CSP – high temperature solar thermal

Concentrated Solar Thermal (CST) technology focuses on generating heat rather than electricity, utilizing concentrated solar energy for various thermal applications. Similar to CSP, CST uses mirrors or lenses to concentrate sunlight. The concentrated sunlight produces high-temperature heat.

CSP is primarily aimed at electricity generation, while CST is focused on utilizing concentrated heat for direct thermal applications. 

Solar CSP systems use mirrors or lenses to concentrate a large area of sunlight onto a small focal point. The concentrated sunlight generates high-temperature heat, which drives a steam turbine connected to a generator, producing electricity. CSP is suited for medium and large-scale power generation and can incorporate thermal energy storage systems, enabling dispatchable power that can be supplied even when sunlight is not available.

One of the primary advantages of CSP is its ability to provide dispatchable power. Thermal energy storage systems allow CSP plants to generate electricity even during cloudy periods or after sunset, ensuring a stable and reliable power supply, something that offers a more stable power for the grid.

Overall, in India and worldwide, the solar CSP technology has simply not been able to scale the way solar PV has.

There are many reasons for this. The initial investment required for CSP plants is much higher than that for solar PV. CSP plants also require significant land and water resources for operation than PV plants. CSP plants are not modular like solar PV is. And unlike the solar PV that converged on one technology (silicon based solar cells), the solar CSP industry is still struggling with multiple technologies with none of them becoming the dominant standard. 

The first CSP plant in India, the 100 MW Noor CSP plant, was commissioned in Rajasthan in 2015. This plant, like a few other CSP projects, is not operational any more.

The Indian government has had CSP in mind right since the start of the National Solar Mission in 2010, but the poor performance of the technology worldwide in the context of its scaling has been a dampener on policies and other initiatives.

It is difficult to predict the growth of the Indian CSP sector given its challenging past.

The future of solar thermal

Around the world, a number of innovators and industry professionals are trying to create significant value from solar thermal, and I think their optimism in the potential that solar thermal presents is justified.

However, given the current status of the technologies around solar thermal and the overall economic structure of solar thermal solutions, in the next ten years I doubt if solar thermal will be able to provide low-carbon energy at scale, be it for heating or power generation, and be it in India or elsewhere in the world.

There is a good chance that most of solar thermal’s contribution during the next decade could continue to be for low temperature industrial and domestic heating applications. Perhaps, medium and high temperature solar thermal could see traction before 2030. I however have serious doubts that we will see a growth of solar CSP (power generation) anytime in the near future.