Solar refrigeration may have applications in both developed and developing countries. Applications in developing countries, such as vaccine storage or large scale food preservation, have been the subject of much research. In developed countries, the main area of interest is air conditioning.
Applications in developing countries
There is a demand for cooling in many parts of the world – where there is no firm electricity supply, and conventional fuels are difficult or expensive to obtain.
Possible refrigeration cycles
There are five classes of cycle that can be used for renewable powered refrigeration systems (Desiccant wheel technology for air conditioning is dealt with later).
- A standard mechanical vapour compression cycle, requiring an electrical input to a hermetically sealed compressor. The electricity is generated by photovoltaic panels. This has the advantage of using off-the-shelf technology, but the disadvantages of high cost and the probable need for an electricity storage sub-system still remain.
- Intermittent adsorption cycles (solid): Adsorption refrigeration cycles rely on the adsorption of a refrigerant gas into an adsorbent at low pressure and subsequent desorption by heating. The adsorbent acts as a ‘chemical compressor’ driven by heat. In its simplest form, an adsorption refrigerator consists of two linked vessels, one of which contains adsorbent and both of which contain refrigerant as shown in Fig. 1 above.
- Intermittent absorption cycles (liquid): These are thermodynamically identical to adsorption systems – but use liquid absorbents rather than solid adsorbents. Typically, the pair used is ammonia-water, but ammonia-NaSCN, methanol-LiBr and other pairs have been used experimentally.
- A continuous absorption cycle with an electrically driven feed pump eliminates the problems of bulk, but if electricity is available to drive a solution feed pump then it could be argued that it would be better to use a conventional vapour compression cycle. The use of a small amount of photovoltaic electricity to drive a feed pump might be justified.
Integrating renewable energy to cold chain: prospering rural India
Since India is a country where agricultural sector is one of the key contributors in the National GDP, the storage and processing of the harvest is very critical. The cooling of majority of fruits and vegetables needs to be done even before it is transported – so as to maintain the freshness and prevent immediate deterioration.
The cold storage facilities for India’s agricultural produce are short by more than 10 million tons. Additionally, the energy expenses account for 28% of costs in cold storages.
A report commissioned by the Planning Commission of India to study the reasons for post-harvest losses in the key agricultural states, like UP and Bihar, points to lack of reliable power supply in these states. It also underlines that the larger cold storages located in city centres have been built primarily to store potatoes. Moreover, the greater the distance between the rural producer and the markets, the greater is the risk of post-harvest deterioration.
Hence, there is a need for self-reliable and sustainable cold storages near the agricultural fields itself. This will reduce the transportation cost and as a result, more farmers will be encouraged to use this facility.
Cold Storage: renewable integration
The cold chain sector is sizable (6,000 units /30,000 metric ton) and fast growing with many key industries critically dependent on it. In the cold chain, we can include renewable energy interventions at various stages to support the development of a self-sustainable model of Green Cold Chain, which requires little or no grid power to drive it. Also, there can be a technological intervention wherein the renewable infrastructure supporting the cold storage facility can feed electricity to the nearby habitat.
This shall safeguard the farmers from unwanted losses due to pilferages, mismanagement of stock and lack of grid supported cold chain infrastructure, also supplying electricity to the villages at the same time.
The renewable energy technologies can be integrated in the existing system or developed in isolation based on the following key factors such as:
Type of stock to be processed: The need of temperature range for various food and produces varies from sub-zero degree to 10°C, and hence, the renewable energy technology shall also vary accordingly to attain the temperature range. Temperature range of various agricultural produces can be seen below at Table 1.
Load requirement for the desired infrastructure: The electrical load requirement for various units as mentioned in Table 2 varies, and the load can vary from 3 to 125 kW. The renewable energy technology can be used in stand-alone mode or to supplement the electricity loads of the existing projects.
There can be multiple solutions like mobile solar powered vans/solar cooled containers for transporting the stock to nearby cold storage, large cold storages driven by solar thermal/solar PV technology. Solar refrigeration engages a system where solar power is used for cooling purposes. Also, renewable energy interventions can be integrated in the existing plants – such as Biomass-Gasifier can be coupled with the diesel gen-sets, this hybrid solution can significantly reduce the cost of fuel.
Renewable energy status: India
The renewable potential in India is estimated to be more than 245 GW with over 100 GW of solar energy potential. Various forms of renewable energies contribute to this massive potential. Many sectors, including food storage/cold chain, can strive for energy security by adopting renewable energy.
Renewable energy is now becoming technologically and economically sound alternative to grid power and can be deployed in far remote places at competitive price and to the required scale.
Renewable energy technological options for cold storage
Solar photovoltaic power pack
Power Pack systems are used to generate electricity for locations where grid is unreachable or the access is expensive.
It is a PV based solar energy system, where solar energy is converted into electrical energy and used for refrigeration – much like conventional methods.
These Solar Power Pack Systems (as illustrated in Fig. 2) can also be used in combination with existing grid for uninterrupted supply of electricity. The power pack consists of Solar PV modules, long life low maintenance batteries, solar inverter-cum-charge controller and suitable hard-ware. The power from solar photovoltaic cells in case of cold storage chamber/container/van is utilised mainly to drive the compressor of the system.
Solar PV system + diesel gen-set hybrid
A solar PV can be coupled with existing DG set to supply electricity for base load; catering to luminous load. During the non-availability of the grid power, the electrical units generated by solar PV can be utilised, hence minimising the energy cost of the infrastructure. Pilot projects are operational at various locations so as to hedge the cost of diesel consumed for base load. At present, there are over 150 cold storages with solar PV set up – where the cost of diesel consumption has gone down steeply with the help of smart controlling and prolonged temperature maintenance.
Biomass gasifier
Biomass gasifier based electricity generating systems are a viable option for decentralised electricity production especially in village areas – where grid is not available and lot of stock is readily available. As seen in Fig. 3 this setup can provide cold storages with electricity even in stand-alone mode.
Solar/biomass co-generation (power and cooling)
In the solar biomass driven cold storage scheme, producer gas from biomass gasifier drives gas engine to produce electricity required to drive the electrical demand of the system. A Vapour Absorption Machine (VAM) is driven on the engine waste heat, which otherwise would have been rejected to atmosphere.
Here, as seen in the block diagram in Fig. 4; the waste heat available from the gasifier engine producing electricity is utilised to power the vapour absorption system.
