Solar Hot Water

Solar hot water systems collect the Sun’s energy to heat water. The systems can provide supplemental hot water to your home domestic hot water system, radiant heating system, or pool heating system. They are more efficient than solar photovoltaic and solar hot air systems.

Basic components

Solar hot water systems are composed of three main components: a solar collector, a distribution system, and an optional heat storage unit. Other accessory elements may include pumps, sensors, filters, valves, and system controls.

Solar collector

transfers heat from the sun to water.

Distribution System

pipes circulate water from the collector to a storage tank, radiant heating system, or pool using a pump.

Heat Storage (optional)

a tank that stores water or thermal energy for distribution at a later time.

 

Solar Collectors

There are three main types of solar hot water collectors: flat plate collectors, evacuated tube collectors, and integral collector storage systems.

Flat plate collectors

consist of glass or plastic sheets, a dark absorber plate, and water pipes with insulation. Sun light heats up the dark plate, which in turn heats the water. These are the most common type of solar collector, and they operate more efficiently during the summer months than in the winter months due to heat loss to the outside air.

Evacuated tube collectors

consist of rows of glass tubes nested with metal pipes and aluminum or copper fins, heat exchanging fluids, and a copper manifold heat exchanger.  The external glass tubes and the metal tubes are separated by a vacuum air space to limit heat loss. Located above all of the pipes there is a copper manifold that serves as a heat exchanger to a home’s hot water system. These systems are the most efficient solar collector type for extremely cold environments, but they are also the most expensive collectors. In Ohio, these collectors work more efficiently than flat plate collectors during the winter months, but less efficiently than them during the summer months.22 Check out “Solar Collector Efficiency Calculator” by BuildItSolar.com to determine the efficiency of these collectors during different operating temperatures. 

Integral collector storage systems (ICS)

also known as “batch” or “bread box” solar water heating units, these systems consist of a big tank or many large diameter tubes that are insulated within a box located outside of a home. They effectively combine the solar collector and storage into one unit. These systems are the cheapest solar water heating system available, but have the lowest efficiency. Since the hot water storage is outside of the conditioned environment, these systems are only viable in places where temperatures do not drop below freezing. For this reason, they’re not widely used in Ohio.18

 

Distribution Systems

These systems can either be active systems, which use pumps, valves, and controls to move water through the system, or they can be passive systems, which do not include any such mechanical elements.5

Direct active circulation

utilizes a pump to move water through the solar collector and directly into the house’s radiant system or a storage tank in the house. There are no heat exchangers associated with these systems.

Indirect active circulation

uses a secondary fluid in the solar collector and exchanges heat to the water used in your home using a heat exchanger. Typically, the storage tank and heat exchanger are both located in a hot water storage tank. This is the most common distribution method for solar hot water heating.

Passive

utilizes natural convective heat transfer to deliver hot water to a storage system. These systems are less efficient than the active systems, but they do have a lower upfront cost. Due to freezing conditions in Ohio, these systems cannot work in the winter time, so the system will need to be flushed to avoid damage to the solar collector.

 

Heat Storage

Energy harnessed from the solar collectors can be stored in a storage tank, masonry mass, or phase changing materials.

Storage Tank

stores hot water. These systems are most common and can be supplied actively or passively. When purchasing a domestic hot water storage tank, consider cost, size, insulation, durability, tank placement, and auxiliary heating fuel type and efficiency. These systems can be used to supply domestic hot water or radiant heating.

Masonry Mass

materials such as concrete or floor tiles have the ability to absorb large amounts of heat, which act as thermal storages. These materials are used in radiant heating systems and can add additional heat storage to the system.

Phase Changing Material (PCM)

PCMs can use a variety of substances that change phases at specific temperatures. It can be used for thermal energy storage in homes if the critical temperatures are close to the set point temperatures for hot water tanks. However, PCMs are not common in residential solar hot water systems.17

 

Selecting a Domestic Solar Hot Water System

Before purchasing a domestic solar hot water system, you’ll first want to understand solar availability of your site, the system’s energy efficiency, heat output, cost and payback, and local codes and regulations associated with your property. Solar hot water for radiant heating design is much more complicated, so it is recommended that you directly contact a contractor for your system design.

Solar Availability

00

Energy Efficiency

For solar water heating efficiency, most panels are rated with a solar energy factor (SEF) and solar fraction (SF). SEF is the ratio of the energy delivered by the solar hot water system to the electrical or gas energy put into the system. Typically, systems have an SEF of around 2 or 3. The SF is the proportion of water heating supplied by the solar system to the overall hot water demand. This means that the SF ranges from 0-1, with 1 indicating that all of the hot water is supplied by the solar hot water system . Learn more at “Estimating the Cost and Energy Efficiency of a Solar Water Heater” by the DOE.

Heat Output

The size of your solar collectors will be determined by the hot water usage of your home and the storage volume of your hot water tank. Typically, this is done by your solar hot water contractor. Check out “Sizing a New Water Heater” by the DOE for more information. 

Cost and Payback

According to homeadvisor.com, a solar hot water heater ranges from $800 to $6,000 and has a payback of 5 to 15 years (corrected for no federal tax incentive). If you are building a new home, the price of a solar water heating system for a 30-year mortgage is typically around $13-$20 per month, and federal income tax deductions reduce the payment by $3-$5 per month. So If you save more than $15 a month on water heating, the system will be profitable. Learn how much your system will cost by visiting “Estimating the Cost and Energy Efficiency of a Solar Water Heater” by the DOE. 

Local Codes and Regulations

Every site has a different set of local zoning codes and regulations. Learn more about these restrictions at “Building Codes and Regulations for Solar Water Heating Systems” by the DOE. 

 

Installation

Solar hot water systems will likely require a contractor since they do use pumps, plumbing, and heat exchanging elements. You can find installation contractors at Green Energy Ohio’s “Renewable Energy Installers” webpage.

 


Resources

1 http://northpiedmont.org/images/meeting/101111/tempsolarashraehandoutgso.pdf - PowerPoint for understanding solar variables (complements of ASHRAE presentation)

2 http://www.iedu.com/Solar/Panels/ - "The Zen of Passive Heating Panel Design" by Morris R. Dovey

A passive design strategy presented by a home owner that implemented an active solar collector approach with natural convective air movement. This website presents a step-by-step assembly of the system.

3 http://www.nrel.gov/docs/fy04osti/34279.pdf - Consumer's guide to implementing solar heating technologies

4 http://www.seia.org/policy/solar-technology/solar-heating-cooling

5 http://energy.gov/energysaver/active-solar-heating  - Solar heating

6 http://energy.gov/energysaver/solar-water-heaters  - Solar water heating

7 http://energy.gov/energysaver/siting-your-solar-water-heating-system - Siting solar water heating system

8 www.energy.gov/energysaver/building-codes-and-regulations-solar-water-heating-systems 
- Building codes for solar water heating systems

9 http://energy.gov/energysaver/heat-exchangers-solar-water-heating-systems - Heat exchangers for solar water heating

10 http://energy.gov/energysaver/estimating-cost-and-energy-efficiency-solar-water-heater - estimating cost of solar water heater

11 http://energy.gov/energysaver/heat-transfer-fluids-solar-water-heating-systems - Heat transfer fluids

12 energy.gov/energysaver/solar-water-heating-system-maintenance-and-repair - Maintenance and repair

13 energy.gov/energysaver/solar-swimming-pool-heaters - swimming pool solar water heating

14 http://www.alternative-energy-tutorials.com/solar-hot-water/ - Great solar source

15 https://www.revisionenergy.com/at-home/solar-hot-water/ - Solar hot water heating examples, cost, and personal design tools. Note, this information is from a supplier.

16 http://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm “Solar Collector Efficiency Calculator” by BuildItSolar.com

Calculate the average efficiency and heat output of a solar thermal hot water system using ambient air temperature, collector temperature, and solar intensity.

17 http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.466.6725&rep=rep1&type=pdf

“Using Phase Change Materials (PCMs) For Space Heating and Cooling in Buildings” by University of South Australia

Research conducted by Dr. Frank Bruno at the Sustainable Energy Centre concerning PCMs and their use in various applications. 

18 http://www.alternative-energy-tutorials.com/solar-hot-water/integral-collector-storage.html

19 “Renewable Energy Installers” by Green Energy Ohio

http://www.greenenergyohio.org/page.cfm?pageId=315

20 http://www.nrel.gov/docs/fy11osti/48986.pdf

21  http://www.greenbuildingadvisor.com/articles/dept/musings/solar-hot-air-collectors

22 http://www.builditsolar.com/Experimental/PopCanVsScreen/PopCanVsScreen.htm

“DIY Solar Air Heating Collectors: Pop Can vs Screen Absorbers” by Build It Solar

Introduction into solar hot air heating followed by simple solar hot air collector instructions, costs, and conclusions.