Solar Domestic Hot Water

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What is It?


A solar domestic hot water system refers to the use use of Solar Collectors to provide the energy needed to heat the water used for showering, sinks and appliances (e.g. dishwashers.) In fact, the system is a hybrid system where the solar (passive) component does most of the work, and only when the solar cannot meet the required water heating demand are high-efficiency mechanical heating methods used to meet the remaining demand. We expect the assistance of mechanical system(s) only during prolonged periods of low solar availability.

A solar domestic hot water system can include:

  • Solar Thermal Collectors ...covered by Solar Thermal Collectors pattern.
  • Domestic Hot Water Storage ... essentially a thermal battery used for preheating the water.
  • Auxiliary Hot Water Tank or Tankless Hot Water heater...only used when there's not enough solar energy or hot water demand is high.

This should only be used after the building's domestic hot water demands have been reduced through installation of high-efficiency end-use devices such as low flow shower heads, automatic shut off faucets, and low usage appliances.

This system is also integrated with the building ventilation system, which is explained in our Geo-Solar Hybrid Heating and Cooling pattern.


  • Also known as: solar hot water, solar water heating, solar water heater design

Why is it Important?


A solar domestic hot water system is important to a building's sustainability because it:

  • Directly connects the building and its occupants to the earth's natural systems (e.g. sun)
  • Reduces energy operating costs because of ultra low energy usage.
  • Includes renewable, high-efficiency, low-carbon components (e.g. solar collectors).
  • Demonstrates an "appropriate" use of technology (only after the natural systems are unable to meet the needs).

When to Use It?


It is appropriate to use solar domestic hot water systems when:

  • The domestic hot water demands have already been reduced through high efficiency fixtures and appliances.
  • While easier to do in new construction, it is possible to do this in renovating an existing building.
  • You have enough hot water demand to justify the commitment of resources.
  • Please look at the "When to Use It" list for Solar Thermal Collectors

Green Garage Use of Solar Domestic Hot Water System


Green Garage - Domestic Hot Water Estimates

Sustainability Goals

The sustainability goals for the solar domestic hot water system are:

  • Meet domestic hot water requirements of the Green Garage.
  • Reduce our domestic hot water energy usage to only 10% of an equivalent commercial building (per ASHRAE data).
  • Connect the building and the occupants to natural systems.
  • Ensure healthy indoor environment- no potential natural gas fumes.
  • Allow components of the system to be bypassed when they don't contribute to these goals.
  • The system should be simple to maintain, adapt and control, and should position the Green Garage for a net-Zero energy future.

Strategy and Conceptual Design

Hybrid Strategy

The major elements of our solar domestic hot water strategy were:

  • Solar Thermal Collectors ...covered by Solar Thermal Collectors pattern.
  • Domestic Hot Water Storage ... essentially a thermal battery used for preheating the water.
  • Auxiliary Hot Water Tank or Tankless Hot Water heater...only used when there's not enough solar energy or hot water demand is high.
  • Put the passive and active components in series with the passive component (i.e. solar collectors) first. Only after the passive components cannot meet the needs does the active turn on and meet the "net" remaining requirement. This reduces the size requirements for the active equipment. It also runs less frequently since it's second in line to the natural system. Both of these reduce energy usage.
  • Select the highest-efficiency, most reliable hot water tank available.
  • Use thermal storage to:
    • Better match energy production with energy demand (e.g. solar heating).
    • Eliminate the peak requirements by allowing peak demands to be met by stored energy, versus needing to be generated on-demand.


Solar Thermal Collectors

The Solar Thermal Collectors component captures the energy from the sun and uses it to heat water running through the collectors. Many of these collectors would be covered in the summer when the collectors generate far more heat than is required for and the domestic hot water.

A complete discussion of our solar thermal collector design is available in our Solar Thermal Collectors pattern.


Pre-heat Thermal Storage

The heated water from the collectors passes through a heat exchanger in the pre-heat thermal storage tank. There the heat is transferred to the potable water in the tank that surrounds the heat exchanger.


Hot Water Tank

The hot water tank draws the pre-heated water from 'Pre-heated Thermal Storage'. We looked first at doing this with a natural gas tankless water heater. This is the most efficient choice. However, we moved away from this choice because:

  • A recent Building Science report indicated problems with tankless water heaters operating correctly when the incoming preheated water temperature is close to the thermostat setting of the water heater. The water heater did not operate consistently.
  • We would need to install all the piping and venting required for the natural gas. Quite an overhead, as there are no other gas appliances.
  • The amount of our hot water demand is quite low.

So we decided to use a small (25 gal) , highly insulated (> R30) electric water heater as the auxiliary hot water heater to insure proper temperatures can be maintained. Water entering this system will already be preheated by the thermal storage tanks.


Integration and Controls Design

Overview

Integrating all the components of the solar domestic hot water system does require significant design effort. Some of the controls will be manual and some will be automated. The key integration areas are:

  • Have the hot water tank on a timer to only heat during 6am - 4pm (adj.)
  • Integrated with the geothermal heat pump is the primary source of heat.
  • The integration of the auxiliary domestic water heater. This would be temperature controlled with the auxiliary heater (electric strip heat) coming on only when needed.
  • From 6am to 7am the domestic hot water has priority over all other heating/cooling needs for heat from the Altherma heat pump.
Key Design Criteria
Heating (Winter) Modes
Geo-Solar Hybrid Heating and Cooling - Shown in Heating Mode (01/22/10)
  • Occupied - Closed
    • Altherma packaged controls the heat source for the domestic water storage tank. When additional heat is called for by the Altherma Domestic Hot Water Tank and the time of day schedule program is during an allowed charging period, 5am to 4pm (adj.) the Altherma heat pump runs and with the internal pump (P-8) and V-5 open to the domestic hot water tank it provides the heat until the setpoint temperature 120F (adj.) is reached and determined by the internal altherma temperature sensor (T-6).
    • Please note, two separate tank temperature sensors are provided with the package to provide the sequencing using staggered setpoints. Refer to the Altherma indoor unit and solar kit installation and operation manuals for additional information.
    • If domestic water supply temperature T-6 drops below its minimum setpoint (105 deg F, adj.), then the auxiliary electric domestic water heater heating element cycles on to maintain its setpoint (105 deg F, adj.) if allowed by the time of day schedule and no other source is available.
    • When temperature rises above auxiliary domestic water heater setpoint (105 deg F, adj.), auxiliary heater cycles off and Altherma controls returns to normal control using solar and heat pump sources.


  • Occupied - Open
    • Same as Occupied - Closed hours heating mode.
  • Unoccupied
    • Same as Occupied – Closed hours for domestic water mixing valve MV-1 control heating mode.
    • Auxiliary domestic water heater and pump P-8 are disabled by time of day schedule during Unoccupied hours.
  • Emergency
    • Domestic water heater and heat exchanger are equipped with temperature and pressure relief valves as required by local codes.
    • Refer to Emergency modes of operation for related subsystems for further detail.



Cooling (Summer) Modes
Geo-Solar Hybrid Heating and Cooling - Shown in Heating Mode (01/22/10)
  • Occupied - Closed
    • Same as heating mode.
  • Occupied - Open
    • Same as heating mode.
  • Unoccupied
    • Same as heating mode.
  • Emergency
    • Same as heating mode.



Shoulder (Spring - Fall) Modes
Geo-Solar Hybrid Heating and Cooling - Shown in Heating Mode (01/22/10)
  • Occupied - Closed
    • Same as heating mode.
  • Occupied - Open
    • Same as heating mode.
  • Unoccupied
    • Same as heating mode.
  • Emergency
    • Same as heating mode.



Controls - Open Design Points
  1. Verify setpoints. Altherma manuals give guidance on setpoints for different control strategies.
  2. Determine if deactivation of auxiliary electric heater during occupied hours can be handled by Altherma controls system.


Supporting Science / Experience

The detailed thermal calculations are shown in pages included here. We thank Laurie Catey for her great contributions to our understanding of how to work with natural systems through a better understanding of the science that describes them.

Proposed Materials / Suppliers

Sampling of tankless water heater manufacturers:

Development Story

The Solar Domestic Hot Water system - Development Story page contains many images and videos documenting the process used at the Green Garage to design, build and operate our Hybrid Ventilation System.

Related Internal Links

Resources


  • Solar Water Heating from Build it Solar. Quite comprehensive.
  • Alan Rushforth's Website...Alan give very practical, proven advice on solar hot water systems for commercial buildings.
  • DOE Department of Energy Site on Solar Hot Water

To Do's

  • Resources (find two more)
  • Short Video
  • Upload images onto Development Story page
  • Image for top of page (Kevin)?
  • Look for more Also Known As... for this page
  • Laurie's data link
  • Why are you listing tankless hot water heaters as a resource when you're not using them? (Peggy)


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Peggy edited this page :)