A few basic principles and considerations govern the design, installation, use and maintenance of batch heaters. IPSWHs are called batch heaters because the heart of the system is the “batch” of water stored in the tank(s). The basic batch heater design is based on a tank or a series of large diameter pipes or tanks. The tank is painted flat black or coated with a selective surface that absorbs solar heat easily and transfers it to the water stored inside. To increase heat collection and reduce heat loss, the tank is enclosed in an insulated box covered on the south-facing side or top with a glazing material.
A "solar water heater in a greenhouse" refers to a system where solar panels are used to heat water, which is then stored in containers within the greenhouse, acting as a thermal mass to radiate heat and maintain a consistent temperature throughout the growing space, particularly during cooler nighttime hours; essentially using the sun's energy to passively heat the greenhouse through stored hot water.
Key points about solar water heaters in greenhouses:
Function:Solar panels absorb sunlight and transfer the heat to circulating water, which is then stored in large containers like barrels or tanks placed inside the greenhouse.
Thermal Mass:The heated water acts as a thermal mass, slowly releasing heat throughout the night, helping to maintain a stable temperature.
Benefits: Sustainable heating: Utilizes renewable solar energy, reducing reliance on traditional heating sources.
Even heat distribution: Water effectively radiates heat throughout the greenhouse.
Cost-effective: Can significantly lower greenhouse heating costs over time.
Important considerations:
Sunlight exposure: The solar panels need unobstructed access to sunlight for optimal efficiency.
Container size and placement: Choose appropriately sized containers and position them strategically within the greenhouse to maximize heat absorption.
Insulation: Proper greenhouse insulation can further enhance heat retention.
Supplemental heating: In colder climates, a backup heating system may be needed on cloudy days.

The most common way to use thermal mass is water barrels, because they have such a high heating capacity. By stacking several 55-gallon drums of water in a greenhouse, the grower can incorporate a lot of thermal mass. Barrels should be stacked where they are in direct sunlight, often on a North wall.
Here is a helpful article:
https://www.motherearthnews.com/sustainable-living/green-homes/build-a-passive-solar-water-heater-zmaz07onzgoe/