Solar Above a Cold Store: 70 kW on a Raised Steel Shed with DG Synchronisation at Arihant Cold Storage, Okhla

Why a cold store suits solar

A cold store is one of the most solar-friendly loads there is. Its compressors run every day of the year, and they work hardest in the afternoon heat, which is exactly when a solar plant produces the most. There are no weekends or holidays when the load disappears.

That makes a cold store different from an office or a single-shift factory. The solar output lands on a load that is always there, so it is used on site rather than exported.

A cold store also has a second concern: its roof. Sun beating on the roof of a refrigerated building adds heat that the compressors must then remove. A raised structure with panels on top shades the building's own roof, so less of that heat reaches it in the first place.

The brief

The job had several parts at once:

• Put a useful solar plant on a roof in a crowded estate with little open space around it
• Lift the array clear of the roof on a structure strong enough for the module and wind loads
• Give the modules a proper slope for sunlight and rain run-off
• Keep the plant working during power cuts, when the cold store runs on its DG
• Never let solar push power back into the generator
• Build it all without interrupting a facility whose stock must stay cold

The solution: a raised steel shed

We engineered a raised steel shed on the roof, to the same design we used at Steadfast in Noida. Steel columns rise about 15 ft at the front and 18 ft at the rear and carry a sheet roof. The Trina modules are fixed on that roof.

The 3 ft difference between the front and rear columns gives the modules their slope towards the sun and lets rainwater and dust run off. A perforated cable tray runs down the middle of the array as a walkway, so the plant can be cleaned and inspected without stepping on any module.

Lifting the array clear of the building's roof has further benefits. It keeps the solar plant out of the way of the tanks and equipment that share the roof, and it shades the roof of the cold store itself.

Building at height in a dense estate

A roofed structure 15 to 18 ft tall on top of a building behaves very differently from a low rooftop frame. Wind speed rises with height, building edges accelerate it, tall columns act as levers, and a roof covered in panels presents a large surface to the wind. The design had to deal with:

• Wind uplift on the roof and panels, and lateral wind pressure on the whole frame
• Bending and sway in the tall columns
• Stability in both directions, along and across the array
• A clear load path from the panels, through the roof framing and columns, down into the building
• Corrosion resistance for a 25 year life
• Safe access for inspection and maintenance

In an estate like Okhla Phase II there is no room to park a crane beside every building. Structural sections, modules, cables and trays went up by hand and hoist. We erected and braced the structure before any module went up, so nothing was left unsecured at height, and a cold store whose stock must stay cold kept working underneath.

What we installed

• Solar array: 70 kW of Trina 665 Wp modules on the sheet roof of the raised steel shed
• Inverter: one Growatt 75 kW grid tie inverter, wall mounted indoors between its DC and AC distribution panels
• Structure: raised steel shed, about 15 ft at the front and 18 ft at the rear, with bracing and a sheet roof
• Cabling: perforated cable tray down the centre of the array, doubling as a maintenance walkway
• Controls: DG synchronisation controller, so the plant runs with either the grid or the DG

The 75 kW inverter is rated a little above the 70 kW array, so it has headroom for the full midday output of the plant.

Running solar alongside the DG

A grid-tied solar inverter takes its voltage and frequency reference from the grid. When the grid fails, anti-islanding protection switches it off within moments. For a cold store that runs on its DG through every outage, that would mean the solar plant sits idle during exactly the hours when power is most expensive.

At Arihant, the plant is synchronised with the DG as well as the grid. It works in four modes:

• Grid available: the inverter synchronises with the utility supply. Solar feeds the refrigeration load and the grid supplies the balance.
• Grid fails: anti-islanding protection disconnects the inverter. The DG starts and builds a stable supply, and the inverter then synchronises with the DG and solar resumes.
• Running on DG: solar output is held within the live load, so the DG always carries a safe minimum share. If the load drops too low, solar output is curtailed or disconnected.
• Grid restored: the plant returns to normal operation on the grid.

Why this matters: if the load suddenly falls while the site is on DG, a solar plant left uncontrolled would try to push power back into the generator. That causes reverse-power trips and unstable voltage and frequency, which is the last thing a cold store wants during an outage. The controller prevents it by tying solar output to the live load and to the generator's minimum safe loading.

Design basis: generation and carbon

The figures below are the plant's design values, not metered results.

• Design specific yield: about 1,600 kWh per kW per year
• Year-1 generation: about 1,12,000 kWh (1.12 lakh units)
• Average generation: about 9,333 kWh a month, or 307 kWh a day
• Generation over 25 years: about 26 lakh kWh, allowing 0.5% module degradation a year
• CO2 avoided: about 78 tonnes in Year 1, and about 1,850 tonnes over 25 years

Carbon figures use an indicative grid emission factor of 0.70 kg CO2 per kWh. Actual generation varies with season, fog, monsoon cloud and soiling. Because the refrigeration load runs all day, every unit is used on site.

Maintaining an elevated plant

A tall frame on a city roof needs more than panel cleaning. Under the five year CAMC, alongside routine electrical checks, we inspect:

• Loose fasteners and deformed bracing members
• Corrosion at welded and bolted joints
• The sheet roof around every panel fixing point
• Sagging or chafed cables along the elevated route
• Dust, bird droppings and debris, which build up quickly in an industrial estate
• The safety of the access route itself

Because the plant runs with the DG, the synchronisation controls and reverse-power protection are tested periodically, not assumed.

What this project shows

• A crowded estate is not a reason to skip solar. Going up, on a raised shed, created the space the plant needed.
• Cold stores are a natural fit: their load runs all day and peaks when the sun is strongest.
• Grid-tied solar can work alongside a DG when the controls are engineered for it, so the plant earns during outages too.

The same approach suits cold stores, food and pharma warehouses and other buildings in dense estates such as Okhla, Naraina, Mayapuri and Wazirpur. Planning solar on a roof that has little room to spare? Our engineering team will assess the roof and the electrical system before recommending anything. Email wecare@mgetenergy.com.

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